It's kind of normal in sports to push as far as you can. I was the filthiest netball player because while I would get called for contact 50% of the time(which meant you had to stand next to the player you contacted until the ball was thrown or shot) it really paid off the other 50% of the time I wasn't called.
I played a tournament where every pitch that was illegal got called and every runner who left early was called. It wasn't as bad for the runners, as they were able to adjust a lot easier than the pitchers. Pitchers who were 16/17 years old who had NEVER been called were being ip'd out of the game. Until each team would get to their 3rd or 4th pitcher for the game. It lasted one game until all the coaches got together and demanded it stop, otherwise they were taking their teams and going home. We weren't playing softball, we were being played by the umpires.
Where this needs to be heavily policed is at the younger years. Don't make it punishable (so no balls are given, no bases are awarded, it's used as a learning experience for the girls) because all that will result in is crying little girls. But if you don't call them when they're starting out, and allow them to continue pitching illegally, it's extreemly unfair to expect them to change their motions after pitching that way for six/eight/ten years.
I don't think it is the umpires place to 'police' the pitchers. The problem is the pitching instructor or the parents ( if they are teaching ) that have incorrectly taught them.
If I speed down the same road everyday, and get pulled over years later..............I don't think my excuse of " well I've never been caught before " will keep me out of a ticket.
I applaud the umps that do make the calls, they are the ones doing their job and making the girls better.