The thing is, preventing someone from getting up - especially someone with any amount of combaat training - involves a lot more than just *standing next to them*. You have to get your weight on top of them, and then usually attack or manipulate one of the limbs. That is not an act that leaves you *out of the way*. It kind of means *being on top of* the target, at least while you're forcing them baack down. So, the Stay Down reaction kind of needs to be imagined as something that interferes with a strike that can be done without risking an ally.