The thing is, it might be that it is allowed on the network, but it might not fit with Mythbusters. The show is characteristically very lighthearted, and doesn't take things too seriously. I haven't seen all of the episodes, but none of the episodes I've seen have used live animal test subjects for anything, and they certainly haven't killed (I know they didn't kill the mouse, but you get my meaning) anything.
Imagine you're watching an episode of mythbusters. They shoot some banana peels filled with plastic explosive at a test dummy to see if potassium improves explosive radius or some **** . Then "Meanwhile, back at the mouse monitoring station" *Brutalised and gored mice*. It doesn't make sense in context.
Hey, maybe my opinion's wrong though
TL;DR: It's not about what's allowed on the network, it's what fits with the show.