Problem is, Geneva IV (and the even older common art. 3 didn't anticipate guerrillas or insurgencies. Geneva IV was designed to ensure the Soviets would treat our soldiers with respect. It wasn't until Vietnam that we saw this kind of insurgency, where we found ourselves as the only party that considers itself bound by them. It's tough to expect soldiers to go along with that, when their battle buddy just got hurt.
The danger to this fire team was splitting their force when they had to assume that this group of foreign fighters (a fair assumption given no complaints from those who owned the house) had probably communicated their situation just prior to beginning their attempted subterfuge.
A split team with prisoners could be targeted based on that communication.