- Abstraction: It makes sense that if you turn something into a toad, you could just walk over and pick it up and stick it in a bag of holding or throw it over a cliff, or cut off its legs and stick 'em on the barby or whatever. But this is where you need to use your imagination. If someone bullrushes the little toad, maybe it's not really an actual bullrush but more like "he runs over and kicks the toad against a rock." Or maybe you can't pick the toad up because zomg it's effing Vecna and even though he's a toad, he's also effing Vecna.

You sort of have to use your imagination about what's really going on past the mechanics.
- Balance: Being able to turn Vecna into a toad and toss him in your bag of holding is not only anti-climactic, it's ridiculous. If it really worked that way, every intelligent creature 1st level or above would find some way to have that power, either as a power, or in a magic item, or something. You'd never get to turn Vecna into a toad because he'd have done it to you first. Or every intelligent creature beyond 1st level would have some kind of magic item that prevents them from being turned into a toad. So at a meta level, there just has to be balance. It's surprising how many editions of D&D we've had to iterate through before this concept was really addressed in any serious way, but this is why you get a power at 1st level that allows you to turn somebody into a toad, yet doesn't instantly become an "I win" button.