[Jan Olligs]

I think there is no rule that explicitely prohibits it, BUT it would be contrary to the spirit of FIRST and you might risk a penalty or similar. Furthermore, try hitting a robot over a distance of 20 ft with a 6 ft high plexi glass barrier 1 ft in front of you. You have to throw the balls in a nice arc and could not even intentionally do it.
My opinion: Do not base your tactics on it or on any other form of ungracious behaviour. You will not make yourself a lot of friends if you try it.
This is just my personal opinion, though, and does not need to be the same as the official or other people's version.