To expand on this, this is actually a benefit of using certain types of MasterCard with Boingo, a provider of WiFi services all over the world. To make use of this service on the plane, and at lots of other Boingo hotspots, you need to spend five minutes on the ground first linking the MasterCard to a Boing account at https://redeem.boingo.com/mcsi/. Your newly-minted Boingo account will then have unlimited usage at Boingo hotspots.

You can only use Boingo on the aircraft serviced by Gogo, not the ones with Panasonic service. You can tell whose servicing the aircraft when you connect to the WiFi - go to the landing portal and look in the browser. If you see pacwisp.net, then your flight was serviced with Panasonic kit, and Boingo service won't work.

This tip also works for any other Boingo hotspot or flight powered by Gogo, including most of the DL fleet.