> If someone you know (a customer, in this case, with an existing relationship) wants to keep paying you, why go out of your way to say no?

Because I don't want to take people's money that I haven't earned. Why do I want to scam people? Blaming them doesn't make it less of a scam.

> unless there is a concrete reason you have to believe otherwise

Here we agree. You need a concrete reason. For example, if your service streams movies and they haven't streamed one in a year, that might be a concrete reason. Sometimes it's ambiguous, and then send them an email.