Well, I am not sure, it's either a bug or they are trying a scheme/scam. Even if you try to sell SBD on the site, there are no people that would really buy your SBD for 21$ while there are sites where you can get 10+ times cheaper. I heard rumors that the site is pretty buggy and had issues in the past.

Later edit: Register is off for me, I think they don't even accept new users.
Coinmarketcap took off hitbtc from the markets section on SBD for the past 3-4 days, but now they are back. SBD was even at ~70$ on HITBTC yesterday.