From the numbers, having Steam support linux games at all is pretty silly from the business perspective.
- Valve's business depends on platforms controlled by Microsoft and Apple.
- Microsoft and Apple have both expanded into the app distribution market, in direct competition with Steam.
- Both companies have implemented code signing policies in their operating systems that could theoretically be used to lock Valve out of their platforms. In fact, these technologies have already been used to lock Valve out of iOS and WinRT.
By supporting Linux, Valve is able to (partially) mitigate the risk of being locked out by their competition. Also, by supporting Linux early, they will be a step ahead of the competition (Origin, Uplay, etc) in the unlikely event that Microsoft does lock them out. Finally, if Valve decides to start manufacturing their own hardware, then they won't have to rely on continued support from Microsoft (who sells their own Xbox console).
What part of that is silly from the business perspective?