Supporting case sensitivity properly, for example using the same capitalization of file and folder names throughout, would work on Windows too it just isn't necessary. In other words, fixing the code for OS X shouldn't break it for Windows.
The only thing that wouldn't work is if the customer wanted to have 2 folders with essentially the same name, such as Steam and steam.
In fairness it's possible that some of the games themselves weren't written properly for a case-sensitive file system. Valve can't be expected to fix that too.