I think their perspective is that they have a say over how their network is accessed/used and being a private (non government) company they are under no obligation to just provide unfettered access.
They may also have the perspective that different devices have higher levels of risk of abuse/missuse and some devices/device types are a higher risk than they are willing to accept and as such they don’t allow them.
As it is currently it is my understanding that these are still private (non government) companies and in turn not a public (government owned/controlled) space/service which gives them quite a bit of freedom to limit access/usage however they want (within some amount of regulation).
I’m not saying that’s how it should be, just how I perceive it to currently be. As such, you may be able to get a librem 5 onto Verizon unofficially and unsupported, but that access could be revoked at any time with no meaningful recourse.