They're not omniscient, but I wouldn't count on that. It wouldn't be all that hard to collect a unique list of URL's and have an intern go through a bunch of them per day. The list could probably be augmented pretty easily with some analysis of the page content to detect fee related text, and they certainly have the technology to make that happen. They can narrow it down pretty quickly too based on what key your using to access the API.

The terms are pretty clear. Better to pay it if your using it and charging for it up front rather than try to get clever and get caught later and potentially have legal fees to pay on top of it IMHO.