[mschuster91]

> Would it be possible for someone to hijack the SIM card in such a device

Depends if it's using a physical SIM card or an eSIM module like Apple does that's entirely provisioned from the device's userland.

> and use it to get free calls all over the world, including long-distance?

Depends on the plan assigned by the vendor / phone company. If it's a data only plan, then no.

> Would that be illegal? Would anyone care? :)

Depends on which jurisdiction you are and what you're doing. A couple of dollars worth of charges on a company that sells millions of "smart" devices? Probably won't even get flagged. A couple hundred dollars for calling a phone sex line or someone on a satellite phone however? That will cause someone to have a look.