Okay, maybe I have to give you the reverse example.
George Lucas and the Star Wars films. Everyone hates that he diddles with it. They don't want him to change it. You can petition him all you like, and I'm sure people have, but he hasn't. Just because thousands of people want the writers for ME3 to rewrite the ending or add in something extra doesn't mean they have to, or that they should. The author is note beholden to a bunch of upset fans. I'd lose all respect for any author who caved to internet petitions to change the ending of his work.
Basically it's the demand of, "We don't like, change it" While yelling loudly in large groups. It's a bad ending, and I doubt any amount of whining and yelling over the internet is going to get Bioware to do anything about it; honestly, I'd lose all respect for them if they did.
Have to? Hardly. Should? Debateable. Entirely subjective.
I think your position is a highly arrogant one. There's evidence that this controversy is actually hurting BioWare - costing them sales and causing sharp price drops. BioWare is a business, not a writer. I'm sure they can afford those lost profits - but then again, in the long term, they might not.
You say you'd lose all respect for a writer who changed something because of "a bunch of upset fans". Which, again, is sort of insulting, considering the popular polls show only 2% of those who vote are happy with the ending, while something like 85% are very unhappy. A few people have tried to dismiss this on grounds that "only" 30 000 people out of a few hundred thousand have voted, but they clearly don't understand how polls or statistics work. It's fairly reasonable to assume that, even if 85% of the total players aren't totally dissatisfied, a large chunk of them are. And suppose they're right? Is it always
wrong to give in to fan pressure, even if you think it over and decide they're right? Like I said, an arrogant position.