These are my personal opinions and thoughts about the subject. Right off the bat I'm against Teeworlds on Steam.
The $100 is a non-issue but looking on it from the Valve perspective, there is no business reason for them why they would put Teeworlds on steam without charging money for it. Teeworlds shouldn't be anywhere near money, DRM and closed-source.
I have don't know how the licensing and NDAs around SteamWorks looks like nowadays so I'm gonna guess here from how these stuff usually works. You are most probably not allowed to redistribute any source that contains SteamWorks stuff. This would turn Teeworlds into a partially open-source game at best, where the real open-source version is not on par with the closed one. This is bad.
Taking Teeworlds to Steam is turning Teeworlds into a commercial product. Teeworlds is, and should remain an open and free software.
Also, does anyone know of a F/OSS game that has been put onto steam?