Agreed. I've been tanking as a Powertech, only at level 28 currently. Most of my groups simply do whatever they want until they die, and then they chill and stick together, let tank get aggro, etc.
I think a few more user-friendly UI additions by Bioware may help out. Most people are probably too used to DBM, Omen, etc. to help them choose targets.
Add-ons do not fix idiocy.
OP, I say let them do this. If they die, it's their fault, not the tank's. If the problem persists, drop group and get non-useless DPS in your group.