I've only awarded 2 PTs from a collapsed scrum that went down after getting pushed back (around 6 weeks apart this season) and in those cases the whole thing went to ratsh!t. This, in my experience, is the norm. The prop under that is continually under pressure is typically either replaced or put in the bin earlier. The "pull the rabbit" YC to the FR player after the scrum goes down, even when you're already warned the skipper about his entire front row, is a rare occurrence (and up until Simon mentioned his example, I've never seen it).
It's not about being "above the law"; it's about the clear and obvious. Props on the end of a scrum that collapses after getting shoved back 4m arent trying to "get one over" on anyone; their scrum is utter garbage and they're trying to survive. They know the penalty try is on, so they get driven back and bail due to a lack of control or looking for safety. I'm the last one here to sympathize with a shitty scrum, but it's important that we dont go throwing fuel on the fire.