Call me crazy but I can't help but think that the large majority of Valve's customers don't care whether the design is open or not.

They probably don't, but they probably had some people internally pushing for this, and seeing how it likely wouldn't cost them much but had the potential to generate a lot of good will and potentially increase demand for their product. Why would they not do it?

Not that it's much of hinderance these days but it will make the knock-offs easier and quicker to produce, and of a higher quality. I know the controllers are assembled in the US[1], don't know if the injection molding is too.

[1] https://www.youtube.com/watch?v=uCgnWqoP4MM

Maybe they don't, but its a huge win for some groups and tends to generate goodwill and news / hype if anyone does anything halfway interesting with it. Valve relies on user trust, there isn't much stopping anyone from using another distribution platform; people have to go out of their way to install steam instead of using the Windows store. Being the 'good guys' is a big part of their business model. I dont want ubisoft managing my DRM.

Also, Valve's highest grossing games are all originally user modifications.