Isn't it the job of the User Agent to ensure the User can interact with the content?
It's a bit odd you could get sent to prison if your website can't be used by e.g. a blind person, but it's perfectly fine if it can't be used by someone without disabilities.
Who or what is the User Agent in this context? The browser for websites?
I'm not sure I quite understand what you are getting at. If you are company with more then 2 million EUR in profit, and you build a website for Transportation tickets, just add proper HTML tags so screenreads work. That is the gist if it.
The browser is the user agent. However browsers are lacking in the accessibility department, so instead we have to do all these weird hacks to make it work for them.
It's a bit odd you could get sent to prison if your website can't be used by e.g. a blind person, but it's perfectly fine if it can't be used by someone without disabilities.