Before "roaming included" and "roaming at a fixed rate" were the norm, the carrier your roamed on would submit billing to your carrier, and your carrier would just put the charges on your bill (including such great items as "daily roamer fee" etc).

Nowadays, the carrier you're roaming on still submits billing to your carrier, but your carrier either doesn't just pass the charges through to you, they either eat the charge as part of their roaming included plan, or they charge your their pre-determined roaming rate

This system would work the same way. Depending on how expensive their roaming rates would be, your carrier would decide whether to include usage in a plane in their plan or charge you extra to recover their costs. Either way, the carrier providing service in the plane is making $$$ whenever someone uses their phone.