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.