The reason Apple does this is because you may be using an app other than the web browser. For example, the only thing you might be doing is syncing your e-mail. In such situations, you would never see the portal page, and your app will mysteriously fail to connect to the Internet.
Therefore, before your app has a chance to access the network, Apple does this for you. It sends out a request to the above URL. If the request gets redirected, then Apple knows there is a portal. It then launches a dialog box, containing Safari, to give you a chance to login.
At my local Starbucks, all web surfing is free. But, Windows presents a captive logon page where you must accept the Terms of Service, but the iPhone doesn’t. I assume the portal detects this URL, and automatically opens up the access-point without doing a redirection. I need to test witha Linux distro in order to figure out what’s going on.