Yeah, that's a great point, but due to the unique "sponsorship" arrangement of Redis, Redis isn't actually allowed to accept money from anybody else for anything.
It's a tradeoff between "stable" income and "potentially unlimited" income, and Redis is still in the "stable" column.
That still relies on the company, or people therein being not quite economically rational: free riding is cheaper. Unless of course everyone does it and the good in question is then underprovisioned. It is certainly worth a try, though!
The thing is, free riding isn't necessarily cheaper for anyone.
When you're depending on the software in question, it being underfunded might mean you're not getting the quality and features that would pay off in multiples for you. You could be sitting on a pile of bugs, or your dependency could be abandoned at your expense. These things happen all the time in open source land and it's a tragedy.
I'm not convinced that freeriding is really rational at all. I would however grant that it's both easy and lazy.
