Though I think there are more important things to get upset about, I will say:
When the casting choices actually start changing an established character (which they sometimes do), it puts people off, because they were advertised an established character and paid money to see said established character, because they LIKE said established character.
Though trailers are starting to because overt when a bait and switch is in progress, there are still those who mistakenly expect respect to the source material.
I'm pretty sure people would complain if they were advertised a veggie burger, paid for a veggie burger but got beef instead, because, a burger is a burger right?
The real question is, why are people trying to change movies, instead of just boycotting them if they don't like the production choices?