Many captive portals don't want to be detected in a separate flow. OS X, iOS, Android, Chrome, Windows 8, etc. all notice if you're running a captive portal, and pop up a separate browsing window: as soon as you can reach the portal, they kill the window and let you get back to your work.
But if the portal was going to redirect you to some ads or other "value-added" content, then they may not want that window to be killed. My former local Barnes and Noble would explicitly whitelist Windows' detection URL, so that they could redirect you to the BN home page instead of to the page you were trying to visit.
And seriously, let's admit it - the "value added" thing is bullshit, and captive portals are mostly either useless (TOS that no one reads anyway) or evil ("value added"). And as I see a few of my cow-orkers working on a captive portal right now, I can't help but think that marketers indeed live inside a strong reality distortion bubble, not realizing that the product they want is making everyone's life worse.
