For getting a bonus of registration on some national exchanges you get a specific bonus, but that one becomes available once you deposit.
Then they are a scam. They already have your money - why do they need more of it?
I had 0.01 btc from a 2013 registration on bitcoin.de . I got an email recently remembering it to me. But now for using it I need to do a 1k euro deposit to get that bonus. Did that, received it and all is fine atm, but for sure I am not going to use their service in the future, but for that 1k euro deposit they won at least half of the bonus ammount in fees.
And there are so many other bonuses that I find pointless to specific but as same. That doesn't mean they are a scam.
What is a scam what the topic creator is saying. There is no legit place where people will ask you to do a deposit in btc exclusevly to prove your identity.
