Anything can be an issue if work hard enough.

This is, conveniently, a decision that discourages their customers from buying hardware from anywhere but themselves an anti-competitive practice which carries little risk of lawsuit, or fine.

It's certainly not a technical problem. We all, mostly, use the same cellular network and other carriers have no trouble supporting devices purchased directly from the vendor.

Yeah, their BYOD plan works great if they sell the phone you're using. My Pixel 7 I got from the Google Store worked just fine with AT&T for two years before I upgraded. I just didn't even consider whether they sold the device or not to be important to the functionality of the phone.