What a ***-***. It is *technically* fair game since it goes yellow, but it really is a ***-headed, rude thing to do. I had that happen to me when I was fighting the worm that drops rdm/sam/pld head seals in Attowha. It came unclaimed for a second at around 85% after CS, and a ranger pops out of nowhere (breaking invis) and shoots it and the ~6 other players break invis. What's worse is it was someone from my ein shell, so I had no choice but to team up with them even though I wanted to call them *** and leave.
I spent time on my pop, 4 panaceas for mudslide and RR items and they came and jacked it. When they started ***storming it and dying cause of mudslide I told them about the panaceas and strategy and the smart *** said "Thatz y we brought mages durr". They wiped ~3-4 times before we finally killed it when I've soloed it before with no deaths.
/end rant
Anyway, people just suck *** sometimes.