You're speaking from a position of how the process is supposed to work, but not from how it actually does work. I think you seriously underestimate how much power an HM can have in the process if they have a little bit of experience.

In practice, the HM absolutely does make the decision. If the candidate is seriously "below the bar", then the BR might strongly assert that they will not be hired. But in reality, most BRs do not actually uphold a very high bar. The BR's main function is to act as a facilitator during the debriefs, but if the HR feels strongly one way or another, the BR will almost always back up the HM.

If an HM wants to hire someone just so they can fire them, all they have to do is find a mediocre candidate that a BR will not feel strongly about, and then during the debrief they just say that they know the needs of their team better than the BR does, and the BR will say "ok, I'll go along with your decision". It's as easy as that.

This is especially so on teams that use the same BR over and over - hiring teams can actually influence which BR they use for loops, so if they find one that consistently disagrees with the HM, they can simply just find a new BR that is more agreeable.