The Tailwind team created one of the most used CSS frameworks ever and charge nothing for it. The framework needs a revenue stream to enable continued investment. Rails has basecamp. Bootstrap had twitter. React has meta. Tailwind has Tailwind UI.
I find it great value. It's a one-off cost comparable to hiring a designer or front-end developer for a day, and you get a bunch of usable components + future upgrades.
Usually HN isn't as negative about revenue streams for open-source developers.