Me personally? Because I love the show. I criticize because I love it, I'm not some kind of girl who just sits around all day complaining about stuff. I'm also not the girl who will just say, "Hey, sweetie, you did a great job!", I won't mindlessly praise something. Because MLP is so exceptional, I've set my bar higher for S4, and this is the only episode so far that has disappointed me.
I do agree that we, as a fandom, have a major tendency to nitpick over pointless, trivial stuff. But I'm also not going to sit by and just shrug when something stupid comes out--the writers and producers and animators of MLP have shown that they can and will produce quality work with excellent scripts; I want more of that.
Also, something that I can't stress enough: Love and Tolerate =/= Blind Acceptance.
We do this all the time with our artists, too. We tell them that, "Hey, that looks awesome! Way better than I could ever do!" when everyone knows it's not true. I like to draw ponies--if I have something wrong with the anatomy, I want someone to say something. I'm also a writer; if someone reads my stories, I want them to say that they're read it, and enjoyed it, but here's something to make it even better. I thrive off input and criticism from my fans, because otherwise I'd never grow.