I was just giving an example of when it wasn't due to a signature.
As you said above, account farming is a thing. I'm sure that the sale of accounts would still be around, though likely not as much, even with paid signatures disabled. There would still then be low quality posts from people trying to boost their accounts to sell.
How do we stop account farming? We do what I suggested. We tighten the grip on any low quality post, irrelevant if they have a signature or not. The only way to do this that I can see is create a solid ruleset on what is and isn't allowed in a post and enforce that ruleset strictly. This would be the only way to deincentivize low quality posting.