Yes, it's right there in the section of the rulebook under Referee's Discretion right after the part where it tells us to make up the rules as we go, and to ignore the rest of the rulebook.
While it's true that the rules in this scenario give A a free pass on any non-USC foul when B is trying to score on a try, I've never seen A foul on purpose in order to prevent a score by B. The sitch is rare enough that I'm sure no coach ever discusses it in practice, and players are simply not thinking that much in reaction to a play like this.
If a team is so well coached on special teams that they have time to discuss this scenario with their players, more power to 'em.