Oh, sorry. Of course they're allowed, the feature was fully implemented and intended by Mojang.
When it comes to such things, remember. Spider spawners in mineshafts were introduced as a challenge to the player rather than an exploitable resource, people just use it as if it was (sometimes) (they still exist as a challnge to players). Animal farms though, they were intended to supply the player with food, so they're legal(and as long a players use them for that, they are. What else could you use an animal farm for?).