My opinion is that guessing why venues put up a portal isn't always that useful. The point is, they
have a reason. In some countries, there are legal requirements to display ToS. While we could try to implement ways around ToS type portals, are we helping the venue be more compliant? In all jurisdictions? (Some laws *require* the ToS is displayed *every time*).
For some locations, they may simply WANT to annoy you! (While also collecting per session fees from Boingo or iPass for the privileged of skipping that annoying portal.) More than likely, however, sites like the one you mentioned are just themselves concerned about liability and want that minimal amount of disclaimer...
As a user, you should either be willing to view the portal, pay for ease of use, or find another network...
David