The mandatory capture if a Knight Canters into a Jumping position has been in the rules since 1930.  I think the idea was (and is) that if a Knight Canters next to an enemy piece that can be jumped, it has reached, in effect, the same position as one where there is an available Jump at the start of a turn, that is, a situation where a capture is mandatory.

I think that it's a good rule, and seems to me to be consistent with the overall conceptual structure of the game.

As a practical matter, it's rare for a player to Canter a Knight into position to Jump, and then be better off if he weren't forced to Jump.  But it is obviously possible.

Good question.

Michael Nolan