"If its yellow, its free game." is what a GM would tell you.
Just because you killed the PH's doesn't mean that the NM is actually yours.
The strategy this person is using is simply this;
Let you kill PH's so he/she can focus on claiming the NM, while you get to worry about the NM AND the PH's.
Personally, when there's people around for an NM, especially hard ones, I let them take care of PH's links - saves me the trouble of possibly losing too much HP/MP for when something pops. >.>
And of course, there's no point in fighting for PH's... I don't particularly like getting /slapped for killing a PH.
However, it DOES suck that people do that. Unfortunately, this game ends up being a "feed people their own medicine" contest. I can't count how many times I've lost an NM because of people doing that. It happened so much that I decided I'd stop killing PH's - I viewed it as giving them an NM they don't deserve.
Some people will be really stubborn about it. When I used to try for O.Kote, there were about 12 people there... They all watched as I killed PH's, then would rush to claim NM. I stopped killing PH's...these people didn't move for like three hours. I received a few angry /emotes and /tells for not killing the PH "for them", but hey, not my problem, right? lol
In all honesty, if you're camping something and see people just letting you kill PH - stop. Let them do the work. They'll either kill PH's, or leave.