I would think this is because you are using an exchange where you do not actually own the coins, you are purely opening a position as to how well they are going to do.
it could be that but think with me if they are only opening positions an not trading really then there is no logic in accepting deposits on EOS.
an example BTC/EOS if they are just opening positions and not trading i would deposit btc do a trade an when i take profit from that i will take profit in btc not in eos and the opposite is aslo true if they are opening positions just like the forex market if i deposit eos and trade the profit will be in eos because this is not an actual trade. regarding that it is useless to accept deposits in eos because if you want to especulate in BTC/EOS you do that with btc
the same way in forex when you deposit dollars then you open a trade in EUR/USD you will not get eur for that trade