From the post, I guess your problem is with pugs (pick up groups) and not pre-made groups where you play with friends.

When you play in pug you have no idea if the healer is actually any good or not. If the tank for example sees his life falling and you are just sitting back spamming holy or whatever it's only natural to cast to cast healing.

You might feel that you have it under control (because you want to cast benediction or whatever) but the person taking the damage doesn't know that. For all he knows you might as well be spamming attack and forgot about healing.

For every good healer that "has everything under control" there are 10 that do not. If you don't want them to cast healing spells just keep their life relatively high (for example around 75%-80%).