If I may jump in, that likely was the reason behind that prohibition. Several ancient texts list several rules about how and when and from whom to feed, and all caution against feeding from shifters, mages, the fae, and essentially anyone who isn't an ordinary mortal. The reasoning given is generally along the lines of "however fun it might be at the time, the consequences will be dire."

Of course, you can imagine how effective a cautioning that is.