Different phones have different capabilities, what's there to explain? SMS is something people pay their phone company for, use their phone number for, follows them when they port their phone number to a new phone company, and works when they send it to somebody using a different phone company. It's obviously part of the telephone system.
(And in actual fact, there are phone companies that do offer SMS service to landline phones.)
> Different phones have different capabilities, what's there to explain?
That your definition of ‘phone’ is meaningless. If phones can have any capability you like, then ‘phone’ doesn’t mean anything.
Once you are playing that game you may as well just declare that social networks can be regulated because they are a ‘capability some things that can also communicate with phones have’, and phones are already regulated.
(And in actual fact, there are phone companies that do offer SMS service to landline phones.)