I have been answering 2 support tickets for more than two months.
They allow the purchase in btc of COX and do not allow the sale when they can not be moved to another exchange. Yobit is stealing user's coin in that way and it's not the only currency they use to cheat. BEWARE THIS IS A SCAM
That's the reason why you have to make a research first before starting to trade that coin in the market sir! Maybe they just restrict the withdrawal for that coin or its maintenance or sometime its subject for delisting.
I agree to you mate that's the reason trading job need a skills before start to trade because trading job is risky, if the people they do not know how to learn and monitor the kind of coins availability into marketcap surely the investment lose, to avoid this kind of trouble just research and monitor before decide to invest.