I don't think that signature campaigns prioritize clients because it would be difficult to know that Bitcointalk member is customer of specific website, especially if cmpaign managed by hired manager who don't have access to database of customers. But I'm sure that managers prioritize potential customers to get hired. Example about degenerate gambler is good. It's likely that in gambling camapign he would be prioritized, even if he isn't best poster, instead of picking good poster who don't have much idea about gambling.
But that's just gambling campaigns. How about exchanges or mixer, who exactly is potential customers of such services. I think in such kind of camapigns participants is chosen solely based on posting quality and where these post is made.
For me, I don't think they need to know who's a user of a specific website, however when it comes to gambling signature campaigns, it's pretty easy to determine where users are spending their money, since they often posting on a specific sites announcement thread.
The reason I don't think they need to know specifically is, they want to convince already gamblers to come to their website. Therefore, if they hire these type of uses to promote their signature, then they're getting the best of both worlds. They're getting their name out there, and the user that they've hired also has a high chance of going to them to spend their earnings. Therefore, gambling signature campaigns might be able to offer higher rates than normal, since their participants are going to come back to them, and spend a bit of what they've earned, and therefore they get that money back.
For other signature campaigns that doesn't involve gambling it's a bit harder. Although, I can think of a few examples that would be a good indicator that they would use their service after advertising it.