They can not like it. No one is saying they can't. If everyone in the world loved it but you, that's OK.
But to predict the movie will flop based on trailers because they themselves don't like the trailers and then to jump to the internet posting memes making fun of this sure-fire flop... and to do so in such a degree that it makes everyone suddenly drop their expecations of the movie... well, that makes them look... bad. They went out of their way to drum up the opposite hype to purposefully hurt the movie. And when the movie does well... there's a lot of sour grapes.
Those who simply said, "eh, doesn't look good, so I won't go" are not the echo chamber I was referring to so glibly as the Interneteratti.