In the US, it’s free on Delta for SkyMiles members, which is in turn free to join. It’s also free for everyone on JSX with no strings, and they actually are the first airline I know of to use StarLink.
Additionally it’s free on most US airlines for T-Mobile customers, but only on devices that actually have T-Mobile SIMs (so not most laptops).
This isn't important; but Delta's "free with SkyMiles" offering is for domestic US travel. For international travel they're still charging $8/hour. Supposedly they may expand the free offering to some flights to Europe though but YMMV.
I have to imagine this comes down to how they provide the Internet connection. Over land they can use cell towers. Over sea they're forced to use satellites.
On AA, the "free one hour for TMO subscribers" through their app doesn't actually check to see if you're using a TMO SIM. It only checks the phone number against their customer list.
After burning up the first free hour I switch to my wife's number, then my kids, etc. It never complains.
Re: T-Mobile - On United you can just set your laptop user agent to a mobile one and sign on with your phone number. Works fine for both the short period and full flight options.
Is there any hope for a future where all airplane wifi is free?
(Maybe if cellphone plans automatically include satellite wifi?)