Earlier quoted context omitted.
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)
It's the "patent pending" part that sucks. They're trying to patent the idea of changing your video card's CLUT (color look-up table) to reduce eye strain, which is a fairly obvious and trivial thing to do once you know that blue light affects melatonin production.
Yep. They care so much about public helath that no one is allowed to be healthier with paying them. Classy move.