There was a patch that introduced this bug. During that patch, every workshop content that got created or updated will only work if you have the DLC due to this bug. I think this was released since then (at least I hope so), but unfortunately, those workshop items that were created during that time will always need the DLC from now on.

Thanks Solais, they need to fix their problematic code instead of just blowing it off and having workshop developers remake their content.
That's just not good business practice.

http://forums.seriouszone.com/showthread.php?t=64586

Unfortunately, this hasn't been fixed. There is a workaround for it, but that requires the content creators to know about it, and, any updates to the game redownload the offending file. I did what I could to get the word out, but...

Thank you Ryason55 for submitting bug reports and finding the work around for the developers.
It would be so much easier for Steam to make their code compliant than for workshop developers to have to do so much for something that is essentially is not their problem to have to fix.