They have rate limits, but they also want to control the nozzle, and not all their users use all their allocation all the time.

In reality, heavy subscription users are subsidized by light subscription users. The rate limits aren't everything.

If agent harnesses other than Claude Code consume more tokens than average, or rather, if users of agent harnesses other than CC consume more tokens than average, well, Anthropic wouldn't be unhappy if those consumers had to pay more for their tokens.

The speculative reasoning I've seen is that they have optimizations in their CC client that reduces their costs. If that's true, I think it's fair that they can limit subscription usage to their client. If you don't want those optimizations and prefer more freedom, use the API.