If someone worked in law enforcement, or other government fields where someone might decide to make an example of them and just the charge is enough to mess up their career, and they had spent time talking about bump stocks online and posted pictures of their rifle with a bump stock on it…
I could see where somebody like that may decide that it would be safer for them to either turn it in and get a receipt for turning it in, or destroy it and document it’s destruction with photos or video.
The internet isn’t as anonymous as some people think (unless you are really paranoid and keep your personal security in mind with every single post you have ever made). On another gun forum, I have made enough references to past jobs, that feds could likely connect the dots and come up with my name, date of birth, social security number, current address, and current place of employment. Heck, yesterday, a fed stopped by during lunch and talked to me for a while - asked how my daughter was doing, if I had finally finished everything with the new house, etc.
I didn’t consider buying a bump stock until I saw what other people had been doing with them on rifles with bipods, tweaking them to make them easier to use and fire accurately. I didn’t get around to buying one before the NRA called for the ATF to take another look at tightening the regulation of bump stocks, and by then it seemed likely that attempting to buy one would involve dealing with another gun buying panic, so I gave up on buying one. If I had bought one, I would probably have been very leery of mentioning it online.