The silliest thing about this is that hitching a ride on dolphins is one of the least severe acts of animal cruelty they'll endure in the game. The obvious "Ooh, I wonder what they drop..." problem aside, people are of course going to trap them and contain them in rubbish little tanks because they look cool.

I'm sure people are going to draw parallels with the parrot situation, but at least in that case there was a tangible "Kids could poison their pet parrot" scenario. I don't think people are going to be able to go out back and hop on the dolphin they keep in their pool because they saw it happen on Minecraft. The worst case scenario would be someone getting inspired to try one of those "swim with the dolphin" places because of Minecraft, which honestly is quite a stretch in the first place and likely something they would do anyway if they were that type of person.