The text that pops up if you hover over the progress bar that is shown (usually 0.6% for new accounts) says something to the effect of:
Your trading volume for the past 30 days is 0.0000000 BTC. If it reaches 500.00000000 BTC, your nominal trading fee will be 0.55%.
It says this regardless of whether or not you have "Buy" selected or "Sell".
If I look at a stock exchange, say Charles Schwab I see:
- http://www.schwab.com/public/schwab/investing/pricing_services
"Online equity trades just $8.95 each"
So Schwab doesn't feel the need to clarify this any further either.