They all do ;-) Have a range, or ranges that is. No single entity has "all" of the possible IP addy's.
The email registration account setup system is by far a much better way to manage accounts.
There are a fair number of alternatives for regulating joining, and possibly separately, posting. The limitation of e-mail account alone is a weak deterrant, because e-mail addy's are easy to get. I deal with hit and runs at Yahoo forums, and eventually went to "validate new poster (based on a post, not just an addy) before permitting a post." It's manageble only because of the low traffic.
By using the clients pop3 isp email for registration, you eliminate all these 3rd party email accounts, and it tells you who the person's isp is, and makes it much easier to track them down should the need arise.
Also, it stops multiple account creation. It can be done here at FR I would think, and would put a stop to these lefty raids.