What's wrong with that? It's commercial software, and they are nice enough to let you use a copy for free. They aren't (AFAIK) doing anything remotely malicious, like grabbing user data or bundling malware. It doesn't add advertising to your computer.
(I'm not sure if f.lux does any update checking, which some people might count as unacceptable)
(I'm not sure if f.lux does any update checking, which some people might count as unacceptable)