The particular 11-man team I've seen this with, the defender would intentionally seek out the A player to engage just to throw him down... his sole interest was engaging and winning the 'battle' he was looking for with the defender. Not the slightest bit interested in actually trying to locate and tackle the ballcarrier if the play was away from his side. And A invariably started getting tired of it and responded.
In what scenario/conditions would action like this rise to the level of being a defensive foul?
Are you serious? I guess it all depends on how much of this nonsense YOU are willing to put up with. The remedy resides in YOUR choice of (a) A not so gentle warning, whispered into a helmet ear hole, (b) depending on what you've witnessed, an UNR flag, (c) if it was a
REALLY CHEAP shot, a one way ticket to the locker room. Guaranteed, you let this "Stuff" slide,
you'll be dealing with a lot more of it.