Most here will say it is marketing.
I disagree however. Value RAM is typically binned at much less strict criteria. What I suspect is that the binning process for "Mac" RAM adheres more tightly to what Apple recommends.
Of course, that data is not available to us, but is likely available to RAM manufacturers.
The non-Mac RAM will likely work, I won't dispute that fact. But we have also seen that it doesn't always work. Going along with the "You only hear about problems on a forum," the rates at which complaints are lobbed against Mac specific RAM is far less.