On both my Windows and Mac machines, I use Chrome which comes with Flash. So I didn't do a system wide install for Flash at all and thus Steam doesn't see that I have it installed anywhere even though I do.

They could be Linux/Mac users who solely boot into Windows to play games.

To get rid of browser crashes, I browse without flash installed, or disabled, whichever is appropriate for the computer in use.

With ad banners in flash, and several per page, all it takes is one poorly written flash app to take down all the browser tabs I've got open.