> the act of learning someone's open ports is what's considered abuse?

Yes, port-scanning a system without the permission of the owner of the system is widely considered abuse. It's conceptually similar to going to an apartment building and knocking on every door to see who's home and who's not.

The act itself isn't very harmful (my home firewall is basically always getting portscanned by somebody or another), but in practice, the reason it's done is as preparation for a more serious attack.