And I'm saying that I disagree with your normally claim. Because normal people CAN remain friends even after rejection.
Just because they CAN, doesn't "normally" mean they DO. Exceptions don't refute the norm, and I already implicitly accounted for the exceptions.
If someone is confessing to another person, they obviously don't want to be "friends", at least if they can help it-- or else they wouldn't have made a confession in the first place. The attraction to the other also precedes the confession.
If they get rejected, then there's an obvious mismatch between what the two parties want from their relationship that's now been made apparent. Even if the confessor "accepts the rejection", it's not expected that they also immediately and thoroughly throw away the feelings that led them to confess in the first place,
don't feel bad for being rejected, or somehow forget the pain of rejection-- especially when they're constantly reminded of it because their friendship was one wherein that rejection took place. They'll be reminded again if and when the other person seeks intimate relationships with others, or whenever they are in intimate relationships with others, or when they get married. Continued friendship is necessarily a "consolation prize" if only in that moment, regardless of how at peace they may be about it-- if it wasn't, then they wouldn't have bothered confessing in the hopes that their relationship would be changed from that of friendship.
The person who rejects the confessor has to deal with the fact that they hurt the other person in rejecting them (even though it's their right to choose who they want to be in an intimate relationship with). The way they look at the other person is significantly changed, since they probably didn't suspect that they harbored those feelings. They can end up considering the motivations of the other party-- both present and past-- as ulterior, and they may even be correct. The worst case scenario is where the rejecting party knowingly leverages the affections of the rejected party for their own convenience.
The reason I charge that the dissolution of friendships in the wake of a rejection is normal, is because maintaining a friendship after a failed confession is obviously an uphill battle given the material matters to reckon with. Both parties need to really want it.
Lastly, not every dissolution is going to be a "torpedoing". Most of the time, such friendships just wane peacefully.