In that specific case, logged in users are still allowed, however you cannot create new accounts when visiting from that range, so you have to already have an account, or go somewhere else to create one.
When anyone can create a large number of sleeper accounts, it isn't really. Its not like you provided ID when you made your account. So the compromise is that if you accured enough reputation you can ask for an exception.
Mobile ips are often blocked because of the sheer amount of spam and they switch so much that its difficult to block individual offenders.