A paid signature is a form of trust in you. The user will click on the signature or prefer to use the service because he trusts you or sees that you provide high-quality posts, so what you ads will be a good thing.
These are the same reasons why advertising companies pay millions to Messi and Ronaldo, so you must try as much as possible to avoid advertising for scam/HYIP, or at least stop advertising for these services whenever you are certain that they are scammers.
My opinion:
The advertisement should be considered as a paid advertisement, not a personal endorsement from the wearer.
However, users should be responsible for ensuring that the signature they are wearing is not a scam.
If not personal endorsement, why encourage others to use it? Advertising is encouraging others to use it.