I agree that it's unfortunate that there's no notification that accounts will be deleted. There used to be a message sent to all new members that they have to post, but then a server change broke that functionality and we have been unable to restore it. I should check with our tech staff to see if that's been fixed in the meanwhile.
As for the behaviour, there are many types of bots. One very common one in forums just creates accounts then plugs in links in the account information and never posts. I'm not sure what the purpose is - either hope forum users notice the account and check out the links, or more likely, they're trying to manipulate the link's SEO by making Google find it on lots of unrelated forums and therefore appear legit. Either way, there are many, many such accounts created every day, and if we don't delete them our member list fills with porn and malware links.
Another issue is that our forum does not allow links to be posted in your very first post. So many spambots create an account, try to post links and get prevented from doing so, and eventually give up, leaving an account behind. There is no way for us to easily find these accounts using forum logs.
Because of both of these behaviours, we find that an account that is created and that doesn't post is almost certainly a bot - we deleted about 1000 non-posting accounts per month, and the amount of real people caught up by that (that we know of) is about 2-3 per year. Almost everyone who joins our forums posts within 24 hours of creating the account anyway.
