The limitation is probably based on trust/honor. If all they are selling you is an HTML file, there's no inherent way for them to track your usage. They could theoretically scour the Internet for any usage of their template, then try to trace that usage back to particular users, but I doubt that would be worth their time or effort.
If the HTML file includes any code, or any
link elements, they could be using a tracker or some sort of pinging system to track your usage. You should be able to look over the template and see if that's the case.
Here are a couple examples of what tracking code could look like:
Google Analytics tracking code (from AnalytiksApp.com):
StatCounter sample tracking code (from StatCounter):