It's not for banned users, it's for innocent users who are wrongfully caught in an ip ban, or who want to use a proxy, like tor.  Some people abuse it to make new accounts after being banned. Paying the fee to register an account does not magically make previous offenses okay, it's still ban evasion.

Spammer, account farmer. Fee is because I banned all your accounts a little while ago, besides one.

If you're sharing the internet with others who browse this forum, or signed up on a public connection that could be why. I've heard that the only reason users have to pay to sign up is because there has been a previous ban on their IP, or that it's a Tor exit node.

Also, are you using a VPN? If you are then that's probably why you're being asked to pay for signing up. Chances are that accounts registered on the VPN's IP have been banned before.

Your IP address has previously been used for evil on this forum, or it is a known proxy/VPN/Tor exit node, so you are required to pay a small fee before you are able to post messages or send PMs. You can still use all of the read-only features without paying.
Your account contains 200.00 units of evil. To atone, you must pay a total of 0.00804568 bitcoins (8.04568 mBTC; 804568 satoshi). Pay to the address 1FmoVtRgPyWWzXbFPkadkisjs9QVE9ex7F. Once you have paid the full amount, wait a few seconds and then reload this page. If the fee is so small that your wallet is unable to send it, you can send any larger amount, though you will not be refunded the difference.
Alternatively, any forum staff member and some other notable members can manually whitelist you. Paying the fee is probably easier/quicker, though.
If you don't have any bitcoins, you can get small amounts of free bitcoins using the sites listed here. It is recommended that you give the free bitcoin sites the address listed above. Do not collect money in your own wallet and then send the bitcoins to the forum -- this will likely result in significant network fees.
If you find any bugs in this system, email pbbugs-...@.... You will be permanently banned if you send email here requesting free whitelisting.