Users agree to a EULA which stipulates how they can and can't use the font. This contract provides the legal basis on which a foundry would pursue someone for suspected misuse. Here's Monotype's EULA for Helvetica Now[0], Section 9 specifically addresses copies and derivative works. From there, it becomes a legal matter.
[0] https://www.fonts.com/font/monotype/helvetica-now/licenses#