It's not that it's strictly against the rules itself, but rather that it's a good indicator that a player might be doing other, possibly related things that are.

For example, probably 90% of the people who talk to about short sessions appear to be doing it because A) they don't like their PK range or B) they're logging in just to spam locate object (or some equivalent). Both of those cases would eventually get the player in trouble.

This is all why nobody has ever been punished for these things without at least a couple warnings.