If you upgrade and pay a one off fee then you can get many more free domains.
Here is some text from their web site:
Really? A "Freemium Domain"? What does that even mean?
Bz.cm offers domain names for $0.00. No catch. No strings. No limits. No purchase required. Just Free. And then we offer Premium upgrades. Want more features? Bells? Whistles? We'll let you pay a little.
So now it's Free plus Premium = Freemium.
"Sure we lose money on every customer... but we make it up in volume."
Bz.cm is not like other Web 2.0 (Web 3.0) companies that give everything away. We actually charge for some stuff...
Take a look at all the wonderful things we are happy to charge you for...
If you would like to see more details click here
There is also an affiliate program where you get an extra free domain for every person that joins under you and 50% of purchases from your referrals.