Hmm, don't seem to have that issue myself. It's crazy, too, as DCSW is actually free. Hmmm.
I'm mystified - I bought one of my modules in a steam sale and had no such issues.
You could try two things:
1st: Get DCS World from Steam, but don't don't download it - at least not all of it (pause the download). Then go and buy the modules you want, taking note of their codes.
2nd: You could try registering DCS World as a non-steam game in your games library.
That's all I can think of. Like I said - had no issues myself, in their last sale, and I'm also not seeing anything to stop me buying the Huey now.
PS - just found from a forum that option 1 above, is how to do it now
http://theairtacticalassaultgroup.com/f ... php?t=5661
also found this:
can confirm that you need only to use the CD Keys from the Steam Library once you purchase them from Steam.
Install DCS World on Steam then get the CD Keys from whatever Modules you choose, then install the DCS Website Downloads for each and install via the Multiplayer cue.
I did this for the Combined Arms, UH-1 Huey, and the P-51 today thanks to this great sale, and installed them as above so there's no need to run Steam at all to enjoy them.
Of course, having to download DCS World, if that's what you have to do, would be not good, if you have a slow internet connection (or limits). But might be worth it for the sale price of the modules, to then put them into your non-steam DCSW.
I think Steam are playing silly buggers (what's new?).