The difference is that they could reveal the intent in their question, not followup.
What if a potential plaintiff in a civil suit against an ambulance company drops in a question or a post without revealing they are fishing for info, get what they are looking for (say, expert witness for the plaintiff), then their lawyer starts emailing the website participant (poster), or subpoena's the website for the poster's real identity? Or an employer trolls to find out if an employee is venting a little too much?
Used to get lawyers calling the jail like that, asking questions as though they were someone but not explicitly identifying themselves. If we asked, they were required to identify themselves, and the call was cancelled at that point but anything we said up to that point was fair for use in court.
Just a matter of courtesy and style, not an enforceable rule, but it could have a chilling effect on the website's freewheeling discussions.