It's free for end users, but he has had a donation link for quite sometime. He also entered into a deal with AgileBits to advertise 1Password (and its integration with HIBP) on the pages where people do the search on his website. So I'm sure he's getting paid for this...or rather reaping the benefits of spending time on HIBP. As others have put it here, he also benefits from the huge PR and the opportunities that come up due to that.

Yet, this has become almost like a public good over the years, and providing it at no (extra) cost than fundamental costs of accessing it is appreciable.