Yeah, but that's still no reason to then jeopardise relations with NJ by taking out their only practical and proven leader.

Taking out Tony on those grounds and doing it exactly the same way would have just sparked another escalating tit-for-tat war, which both families had just agreed to end.

Don't forget Phil was on the lam, and had until that point been very difficult to find; if they pass up the opportunity to whack him there and then, who knows when he'll next be found. Phil no doubt thinks family are off-limits, which is would account for why he's even with them at his time of death. (Imagine Tony putting Carmela, AJ and Meadow through the same risks...)