To help drum up the hype ahead of the launch of Half-Life: Alyx in March, Valve has partnered with Steam to offer gamers an unbeatabale deal. For the next two months, anyone with a Steam account can download and play each of all previous Half-Life titles for free to help refamiliarize gamers with the franchise or attract new fans.
https://www.notebookcheck.net/Valve-s-Half-Life-series-is-free-on-Steam-for-the-next-two-months.451017.0.html
Valve has not partnered with steam. It owns Steam!!!!