I seriously don't understand why Derpy Hooves was censored if Snips and Snails act like complete knobs whenever they appear. Derpy only made one major appearance in the show once, and people found her an offence instead of Snips and Snails, who've been acting like knobs since their first appearance?
Tara Strong explained why.
@ 24:30: "You're not alone The Hub loves Derpy too, but the problem was that some community members complained, I mean really grown up ponies, that it hurt their feelings or was insensitive, and you can't do that. And then the community came out and saying "well taking it off is hurtful", so they're sort of stuck. But The Hub is with you and loves Derpy and loves you too and I love her too, and I think it's such a great story where she came from, how it was an accident and it's kind of- they're in a tough place because you can't be hurting people's feeling and you can't hurt- if it's offensive to somebody then you have to take it down, that's usually the rule of thumb. So there are members of the community that find it offensive so they're sort of tied. Just know that The Hub's with you, they're not ignoring you, they're just sort of trying to make everybody happy and it's not an easy position. Does that make sense?"
tl;dr - Someone got offended, told The Hub/Hasbro, their hands were tied/had to do it.
She wasn't fully censored, if she was she would have been completely removed, had no lines, and wouldn't have ever been seen again.
Snips and Snails are just slow and stupid. Derpy did a little more than that (on top of which, the word derpy is commonly used offensively), and I suppose since she was more popular it was more of a shake.
If you watched the scene without prior knowledge, it looks like Rainbow Dash is calling a random pegasus derpy (handicapped/stupid/insult, not by her name Derpy) and said pegasus then bumps and causes more damage because of her "derpy"ness. From a bystanders viewpoint, I can see why it would be offensive. Snips and Snails are designed to be slow and stupid, no one insults it though.
