That’s kind of the point. Even if users can choose which model to use (and apparently the default is the largest one), they could still say (For roughly the same cost): your Opus quota is X, your Haiku quota is Y, go ham. We’ll throttle you when you hit the limit.
But they don't want the subscription to be quota'd like that. The API automatically does that though, as different models use different amounts of tokens when generating responses, and the billing is per token. And quite literally is having the user account for the actual costs of usage, which is the thing said users are trying to avoid, on their own terms, and getting upset about when they aren't.