It's because they see fan-made videos like the one after the jump, in which Pinkie Pie beats her friends to death for not smiling enough, and realize some people out there cannot be trusted not to modify the game in disturbing ways.
I understand where they're coming from on that. And yet I keep watching that video that's after the jump. And I would totally play a game version. But you can trust me not to be violent and weird - the rest of you, I'm soooooo not sure about.
h/t andre morelo