I think it's 'malpractice' risk. Like, if a lawyer give bad legal advice on a forum, and someone reads it and follows it and then loses a ton of money because of it, and then the reader sues the lawyer for malpractice.
The plaintiff would never win the case. But lawyers like to avoid being the target of malpractice claims, even if they're super-weak / spurious / whatever.
I think that is the situation.
As a lawyer, I personally don't mind writing about the law or answering general questions on the internet. I wouldn't answer someone's specific questions about their personal legal situation though (without an engagement letter).
I'm not really familiar with how it works and I'd imagine there's others on HN who aren't as well.