I get your point - they might count on the user not using their full quota they're officially allowed to use (and if that's the case, Anthropic is not losing money). But then still - IF the user used the whole quota, Anthropic loses.. so what's advertised is not actually honest.
For me, flat rates are simply unfair either ways - if I'm not using the product much, I'm overpaying (and they're ok with that), otherwise it magically turns out that it's no longer ok when I actually want to utilize what I paid for :)
At any rate, they offer the option to be billed exactly on your usage via the API. But if you tell the average person the service costs $x/million tokens they will have no idea how much that actually costs, they won't know what a token is or how much their employees are likely to use. While $30/user/month is something they can easily budget for and get approved.