Harper was cut because of his mistake.
It's possible. It's also possible you're wrong. There's no way you or I have the access to make that conclusion definitively.
Players get cut all the time, sometimes for making big mistakes, sometimes for roster management, sometimes to take a look at a new face, sometimes for a million other reasons. We don't always know all of the details that go into the decisions.
What we do know is that it wasn't the first time Harper was cut. Harper made the final 53 last year out of training camp and was active for the first game, playing 12 snaps. Then he got cut and re-signed. Was it punishment? Who knows? They would trade for Keshawn Martin shortly after the cut though.
Then he was promoted back to the 53 after Jules got hurt, played in 2 games including the Denver game, then got cut and re-signed to the practice squad. It could have been a punishment, as they picked up WR Damaris Johnson that week. Or it could have been opportunity, as RB Trey Williams was put on waivers that week by Dallas and we got him.
4 weeks later, he got promoted again and stayed on the roster into the playoffs as well, then stayed with us until final cuts.
Maybe all 3 cuts were for roster reasons. Maybe all 3 cuts were punishment for mistakes, some we didn't see. Maybe there's a mix of reasons for the cuts. We won't know with any certainty unless BB want to share.
But I don't see a history of BB penalizing players with cuts for making rookie mistakes. BB hasn't shown anything to make me think he wants to be like Bruce Arians, trying to look like a bad-ass on TV by embarrassing some kid for a parking violation.