It seems proprietary because there are ad links disguised as documentation links. You can't click "Components" anywhere on TailwindCSS.com or you'll end up on TailwindUI.com pricing page.
It's fine that they want to make money but it's very confusing for a new user. Ultimately, they might make more money by dropping the shady links if it's turning away enough new users.
s/Components/Tailwind UI/g