If you read what the article says about what exactly is to be blocked, you'll see that it has a lot more in common with your certificate-pased proposal than the first amendment violation you inferred from the headline.
Carriers are being instructed to not deliver SMS purporting to originate from a phone number that carriers know is not capable of sending SMS. They're not being told to discriminate based on the content of the message or even based on the sender, except when the metadata about who sent the message has obviously been faked. Cryptographic signing is an obvious next step, but also unnecessary before even the most basic filtering of invalid spoofed data has been implemented.
I'm not well versed in telephony, but I've always wondered why after a century we still don't have something as simple as a three way handshake for calls/messages?
Carriers are being instructed to not deliver SMS purporting to originate from a phone number that carriers know is not capable of sending SMS. They're not being told to discriminate based on the content of the message or even based on the sender, except when the metadata about who sent the message has obviously been faked. Cryptographic signing is an obvious next step, but also unnecessary before even the most basic filtering of invalid spoofed data has been implemented.