Lurkers are such an integral part of Internet culture that I can't imagine a board without them, regardless of subject. It's very rare that they mean any harm. Sure, many of the lurkers here are active Dubs within the hierarchy, but the majority are likely faders and the curious who want to see what life on the "outside" is like.
The question is then WHY SIGN ON? You can read almost everything here without having to do that... Just sayin...
Lots of reasons. They might want to save their preferred username to use in the future. They might want to send PMs. They might want to read posts in the members section. Whether they register or not, they can still read your posts here (unless you're posting only in the members section), so who cares if they sign up? Makes no difference to you either way.
If you're concerned about people reading your posts, then be careful with what you post. Good general advice for everyone.