A) Even on HN the temptation has come up. e.g. some comments in posts about ransomware make a similar argument for transparently damaging and self-serving actions. Three letter agencies with much more power and ability probably had people making the same arguments.

B) The payoff was extremely low compared to the possibilities. Either whomever did this was unaware of the possibilities or not really interested in a major hack. Perhaps the idea was that if this actually works, the damage isn't so big, aside from embarrassing the Linux kernel team, and when the team noticed they'd tighten up.