It's extremely hard on their front legs, and it makes them very unbalanced while they're bowing. They can easily strain the muscles or tendons in their front legs or shoulders, or even fall over.
Teach them to do it like this:
It's SO easy. All you have to do is lift their front leg like you're going to pick their hoof, and then do exactly what you're doing now! Just hold their leg bent while they go down. After you do this for a while, they'll do it by themselves. It's really not that difficult, and your horse will thank you. He will also be more willing to perform the trick.