It's because operators are deathly afraid of removing the glass. And, think of it from their perspective: they would rather have 20 individuals each drop a dollar in the machine and be disappointed than one guy dropping $20. In both cases, the op gets the same money but in the latter case he has to do more work. By the time he has driven up to the location of the machine, he is already thinking "job half done" before even removing the locks.

I say this since this has been my experience in seeing pins on location - they are ALWAYS filthy and partially broken. It's also why video killed the pinball machine: ops did the "same" amount of upkeep (none) yet video games were still playable from a user perspective.

Cheers,
Mike