It would be amusing to see browsers get pressured to not implement certain features for them to access the web, rather than the other way around. And it would be ironic for it to be a DRM feature.
Is there any reason at all that websites can't do this?
Of course, just check for the existence of the attestation API described by the spec.
> Is there any reason at all that websites can't do this?
Because it would be an extremely dumb move regarding reach. Banning WEI-enabled user agents means you're banning over 80% of your traffic. This might be okay for a small blog or if the website owner is big into activism, but if you have a commercial interest you absolutely can't do this. Plus, blocking WEI-enabled user agents only hurts the user in the end; Google won't walk back on such an aggressively-pushed feature simply because some site refuses to serve Chrome users.
Unless you are a de facto monopoly. Google can mandate WEI because they are a monopoly, other monopolies can do the opposite.
Sadly, it is hard to imagine a monopolic website that both is used day to day and does not meaningfully benefit from WEI. Banks, utilities, etc. are used too rarely to matter.
So then, by extension, if a particular company wants to mandate WEI, then that's their prerogative but I won't be a customer of theirs for long.