I never pass judgment on parenting decisions, and don't always practice what I preach! But I suspect these are some of the concerns:
There is always (always!) another skill to learn, and for some period of time the gymnast will be 'close' to each of these new skills. It's a slippery -- and never ending -- slope.
While a young child might be happy with a sticker, bigger kids and bigger skills eventually want bigger prizes. I've personally seen parents offer incentives of leotards and even pets!
It might also suggest that the gymnast won't give their best effort without an incentive, which is most likely not the case (but may become true if they get in the habit of expecting a material incentive).
Parent-prizes increase the risk of over involved parents? Not always, but maybe....
Personally, our HC has been known to chastise parents who offer material incentives; she believes the effort (that oomph you refer to) must come from within.