[appleflaxen]

Thanks for the explanation! That makes sense.

So... why does the blog post argue that it's unenforceable?

My understanding is that they are saying "you can't use copyright to enforce something that isn't part of a creative work".

If that's an accurate version of their argument, then why not? The agreement is a contract with copyright as one side of the equation, and some behavior on the other side. Why wouldn't you be able offer a license for a creative work contingent upon things unrelated to a derivative work?