These are not proxies but they are blocking the way between you and the server you want to access.
A "captive portal" is one of these systems that are popular to use in hotels, airports and for other sorts of network access to a larger audience. The portal will "capture" all network traffic and redirect you to a login web page until you have either clicked OK and verified that you have read their conditions or perhaps even made sure that you have paid plenty of money for the right to use the network.
curl's traffic will of course also captured by such portals and often the best way is to use a browser to accept the conditions and "get rid of" the portal since from then on they often allow all other traffic originating from that same machine (MAC address) for a period of time.
Most often you can use curl too to submit that affirmation, if you just figure out how to submit the form and what fields to include in it. If this is something you end up doing many times, it may be worth exploring.