DISCLAIMER: I haven't played ME3. This shouldn't surprise you, I wasn't planning on it until it was super-discounted at best, and considering how much I hated using Origin for ME2 when it was _free_, it was gonna be a long time coming. I have been roughly told what happens at the end and I know a few events mid-story. Please don't feel the need to tell me any more. :) I don't really care much about spoilers, but at the same time, I DON'T CARE about mass effect and want to talk about stuff I do care about.
Is it right for consumers to be enraged by an 'unsatisfying' ending to a work of fiction?
Endings are hard. Many, many good stories are let down by disappointing endings. Most of the time, you shrug and accept it, or write fanfic for how it 'should' have ended, or simply invoke CanonDiscontinuity and claim it never happened.
The amount of rage tends to, in my experience, vary based on how much the ending's "fail" goes against what you liked out of the series in total. Many stories sort of peter out at the end, and that's sad, but it's understandable, it doesn't make you as the consumer feel INSULTED, just disappointed. The story ran out of steam somewhere. It happens. We can sympathise.
Then you get cases where someone who isn't the creator dislikes the ending and forces it to be altered before release, even if the new ending is jarringly disconnected from the work as a whole. Two infamous cases are the play "The Doll's House", which in early performances after showing the complete breakdown of a marriage, had the wife... decide to stay with her husband because Divorce Is Bad. Playwright was not happy. I don't think even audiences were happy. Progress marches forward and real ending was restored. Another, more recent example is the movie "I Am Legend", which spent the whole movie building up to a specific realisation, only for some exec to decide they didn't like the realisation, thus cutting off the movie right before it happens and going down an entirely different route. Leaving all the foreshadowing untouched. The end result is a movie that's vaguely watchable as some sort of sad action flick but completely lost its thematic integrity, or any relation to the title of the film. The "real" ending was shipped on DVD anyway.
This sort of ending-censorship tends to provoke anger because it feels patronising. The execs are saying that we can't "handle" the real ending. They think we are too X to appreciate whatever was there to begin with. It is therefore reasonable that the reaction is to argue with it, to insist that actually we are just fine with the real ending, why are you treating us like this?
Not all ending changes are because of that. Sometimes an author gets rather worn out by the end of a story and decides to kill everyone off for shocks, or because they're tired of writing these bozoes and want to stop, or because they're depressed and want to demonstrate the futility of life, and so on. There's at least one well-known movie that was originally meant to end with the main characters dying for no real reason, and the execs stepped in and said "WTF?" and talked them out of it. Movie (and series) is better for it. But sometimes it doesn't get caught, and Rocks Fall Everyone Dies.
If the reason was that the author was depressed, many fans can understand that and be upset at the result but not filled with anger. If the reason was that the writer hated the fans and wanted them to go away and stop making em write this crap, is it not reasonable that the fans feel a bit insulted by this? :) Especially if they were paying good money to be insulted?
Special benefits go to the category of bad ending where the writers not only introduce a sudden and jarring change, but drastically undermine the entire point of the work at the same time. My case in point is the ending of the Xena TV show. After however-many series of working for redemption and doing GOOD to balance out the evil of her past and showing how just dying would be taking the easy way out and struggling to do good is a much harder and nobler thing, at the very end she... decides that actually she has to die for her sins. SO APPARENTLY THE WHOLE SERIES WAS A WASTE OF TIME THEN. Rage does not begin to describe the reaction. Fans felt personally insulted, because everything the show was about, everything they'd been drawn in to watch, they were suddenly being told was worthless. Apparently the producers were surprised by this. What were they smoking? (This was later officially declared a canon discontinuity - at least as far as the ongoing comic book series goes. That Never Happened.)
Another great way to piss the consumer off? Intentionally fail to resolve the plot at all and leave it hanging for the next installment which you intend to demand more money for. There's a balancing act here. If you're writing a series, or you think you might ever write a sequel, you don't want to tie up EVERY loose end. But at the same time, each installment needs to come to some sort of conclusion.
Imagine if you had been playing ME2 and just entered the big nasty scary relay and got a tense scene of hurtling through super-space and then - you got a black screen with 'Please insert $60 to continue'. You'd need to buy a new monitor, because you would have just punched a hole in yours.
I threw a book across the room once and swore never to read anything by the author again because it ended with absolutely everything the heroes had done up to that point having been utterly pointless, plz buy the next book. No. Screw you.
Let's put it this way. Epic stories are designed to stimulate the emotions. If you work up the reader's emotions and cut them off in a very badly-resolved way, you should not be surprised that they go crazy.
Now, do players have the "right" to "force" developers to make things end better? No, not really, and the idea of lawsuits to get your money back from a shitty ending is just stupid IMO. On the other hand, there are circumstances where I feel it's quite reasonable to be angry and insulted by what you've just played/read/watched. And if you feel insulted, obviously you should feel free to express that.