The difference is the fact that the tadpoles can harbor diseases and parasites that a feeder from a LFS could never have. They come from the wild, and most likely have diseases that a tropical fish has absolutely no resistance to. The possibility of infection from a wild food source has to be much higher than that of farm raised feeders. You could destroy an entire tank of fish without even knowing what happened, because the amount of potential diseases is much higher and it would be much harder to diagnose a single one to treat. You might as well take a bucket full of disgusting brown pond water and dump it in your tank. The amount of bad things that you are introducing would be about equal.
Feeding wild tadpoles that you don't even know the species of is FAR more risky than dumping in a whole bag of feeder goldfish from a LFS.
Like I said, I wouldn't do it. But if you want to, nobody can stop you.