For those who can't do math, 2 cents/100 shares is .0002/share, not .002.
To the topic, many shops offer this deal because they have an agreement with their clearing for a fixed commission for trading a specific stock. If you get this deal, you are limited to what they tell you to trade, which is what they guy next to you trades and the guy across from you trades. Benefit to the firm is that these trades tend to cross each other so that the firm exposure is kept to a minimum.
If you want the freedom to trade stuff like GOOG or CME or GS, etc, you would not get that teaser rate you mention.