Like I said,
unless they subpoena the forum but the BFL labs case is a little different as they were probably trying to find out information about the people behind it. Also, like I said, err on the side of caution if you're worried. Law Enforcement could always decide to go after people who were advertising a service they deem illegal but unless it's a ponzi or some other scam then I think the chances of them wasting time trying to find out who was advertising a service on a message board would be slim though obviously not impossible.
I've see The US doing it with 2 or 3 huge MLM or HYIPs scams such as Genius Funds 10-15 years ago. Giving trouble to people they considered promoters affiliated. I'm talking about scams but it doesn't matter, I mean the point is they can decide you're part of the scheme
I believe those were actual employees, and not someone just advertising the services. They would've had reason to believe that x belonged to y, and that would've been included in the subpeona. Like LoyceV said; the advertisement industry would be absolutely dead if advertisers were held responsible for advertising scams. However, if you advertise an illegal service, that could potentially land you in some trouble. Since a lot of the campaigns users advertise here are legitimate businesses that have been operating for years, this isn't likely to be a concern for many.
They can and will go after Ponzi promoters etc. A fair few youtubers ended up getting into trouble for promoting these things to their audience even though they weren't directly involved they were making money from their referrals etc and spreading the scam to a larger audience. I think even some high profile celebs like Floyd Mayweather and Kim Kardashian got into trouble for promoting pump and dumps and unregistered securities related to crypto coins as well. Those things are a little different to promoting other services but the legality of such will vary from region to region. Gambling sites are illegal in many counties for instance. I'm sure even bitcoin is illegal to use in many countries too.