If the promise boils down to the blog announcement on their blog, the line is 'all Tesla vehicles produced in our factory – including Model 3 – will have the hardware needed for full self-driving capability at a safety level substantially greater than that of a human driver.'
IMO (not a lawyer), it feels like they didn't tell you it would be the latest, greatest, continually supported in perpetuity, never changing hardware. They told you it had hardware that could support a full self-driving system that is safer than a human. Hardware that they do not support now.
I haven't interacted with any Tesla sales folks, but I feel like if you asked about a commitment like this, even before this subscription service, no one is going to commit to that.
They made it clear it's up to you to purchase the self-driving software, and that is where all the magic is. And to boot, the available agreement when people purchased the car is still on the table; if you give them $10k, they will give you hardware. If you want the subscription, you need to have/acquire the supported hardware.
If they had a contract like a phone (which dislike that business model) where you commit to X months, I could see it. But I too wouldn't want to be handing these out to have people cancel after a month.