Makes perfect sense to me, it prevents the more malicious users from registering while the genuine users using an IP address which has previously been associated with these malicious users will have to pay a small fee. Okay, its not a perfect solution, but it has been proven to work over the years.
Here's what theymos said in the other thread if you haven't seen it:
Only abused IP ranges have to pay a fee. The fee is an anti-abuse measure, and it is not meant to be (and isn't) much of a source of revenue.
If any very-veteran members want to volunteer, I can give you the ability to whitelist users. (All Staff can already whitelist users, as well.) Perhaps then someone could create a topic like "List of email addresses for people who can whitelist you for free", which I could link-to on the evil fee page. However, these IPs have evil for a reason. If it was sufficient for them to just solve a captcha or something, I'd have them do that.