Welcome to the forums. It is not common but every now and then I hear of a chameleon that will drink from a bowl. I have a Gracefull and would love to see pictures of yours.
They may have learned that dishes are a source of water if they were kept incorrectly before you got them. I have seen very thirsty chams in poor setups drink out of dishes...because no one provided more natural sources of drinking water (dripping foliage, drippers, etc). So, now they have learned. As long as the cage humidity is correct and their dishes are kept free of dead feeder, poop, etc. it won't hurt them.