I guess crypto currency/assets exchanges just followed Gox model since nobody complains. 0.2% sounds cheap but actually is pretty expensive for the current price and volatility of btc.
or maybe they are not confident to handle volume so opt to take big fee.
VertEx charges a full 3% on every trade in either direction. Where are you getting the 0.2% rate? Is that what mtgox charges? That's less than 10% of the rate I'm paying at VertEx!
I agree that it may be an attempt to reduce volume due to capacity limits on the servers, etc... However, VertEx just did a big upgrade on their servers less than a week ago.