If you're not hindering the group in any way and you're not breaking any rules that you were informed of and agreed to when you joined, it's none of their business what you do in your spare time.

I've always shaken my head at the chest thing. Loot lockout is fine, but I've never understood the chest reduction just because someone has cleared. The cleared person can't get any of the loot. But the game actively discourages you from helping your friends because just by joining them to help out you will hurt them.