And does the http://www.amazon.com/gp/mas/get-appstore/android have the same exact apps as on Android Market?
Followup from How to download apps without a wireless data plan
As long as your device is allowed to install apps from Unknown Sources (Check Settings), there's no need of data plan to install Amazon AppStore. Carriers don't (and, can't) lock every third-party app to promote their data plan. In fact, it may become a legal issue for them.
Amazon AppStore isn't exact copy of Android Market, but it has really a large no. of app collection, thanks to e-commerce giant. Its the only app store for Android which could compete with official Android Market. In fact, its a threat for Android Market.