Ok, lets do some maths:
fees 0.6%:
1btc sold at $1000 = 0.006 * 1000 = $6 fee
1btc sold at $100 = 0.006 * 100 = $0.6 fee
Therefore, at higher dollar value of bitcoin, MtGox makes more per transaction and hence there is a conflict of interests?
Cheers, Paul.
First off, the more you trade, the lower your fees.
Secondly, 1 BTC = 1 BTC. I believe Mt. Gox has said at some point that they do not sell their BTC for USD. If I have time, I'll try to find the quote.
Thirdly, as others have mentioned, you are ignoring the volume (the size and number of transactions). It does Mt. Gox no good to make more per transaction if there are fewer of them and/or they are for smaller amounts.