They do the same for many more coins where they have the particular coin's wallet listed as under management or something like that but do allow it's trading and this practice is not at all good and by this they cheat people as it has also happened to me in past as I was searching for arbitrage opportunity and found one between yobit and poloniex which was about 33% but when I brought that coin on yobit and requested the withdrawal then there was written that wallet is under maintainance so all my hardwork was in vein.