Maybe not yet, but probabilities would indicate that eventually an umpire will suffer a serious injury from these premeditated beanings. If they continue, its inevitable...and why not? Next time I want to seriously harm someone with a small object, I'm going to drag them onto a field, peg 'em, and call it a baseball game...that way its legal!
RE: umpire associations, the ever-present grub for the Smitty-buck is a serious impediment to concerted action, just as the 'win now' mentality helped Tark the Shark, et al, gain repeated employment despite their shoddy characters and repeated ethical violations (hi, Slick Rick!).
You think they'll agree on this type of stop-work action? Someone will always take the games if they are paying....like that old saw, 'We've established what you are, now we are discussing the price."