No way to tell for sure, but throwing a tennis ball is the same as underload throwing and that was shown by Coop DeRenne's studies to be as or more effective than throwing overweight balls.
I remember doing that as a kid and I will say that I had more of a tendency to hurt my arm throwing a tennis ball. You can get your arm moving faster with the much lighter ball and therefore the decelleration loads are greater. However, if you are really only throwing at 50 to 75% then it probably isn't helping with velocity or hurting your arm and you've just picked up the velocity due to growth, maturity, etc.
So, yes it could be contributing to more velocity and yes it could be dangerous for your arm. It would probably be better to work with about a 4 ounce ball instead for doing your underload throwing, but then again that wouldn't be as much fun as playing wall ball.