[AlexandrB]

IANAL, but my understanding is that the DMCA makes it illegal to distribute information or devices that would let you break HDMI encryption. But it's not illegal to "tamper" with your own device.

For automobile manufacturers to use the DMCA in this way they would have to make a case for some kind of copyright infringement that would be possible if the cellular modem was removed. Not impossible, but it would be a stretch for now.

[GravitasFailure]

A few manufacturers have tried using the DMCA in an attempt to quash non OEM parts for repairs and modifications, with John Deere being something of a poster child. I believe it's been at least partially quashed for the consumer market, but we'll see how long that lasts.