Electronic Arts released the following games as freeware for these reasons:
Tiberian Dawn - to promote the release of Tiberium Wars (2007)
Red Alert 1 - to promote the release of Red Alert 3 (2008)
Tiberian Sun - to promote the release of Tiberian Twilight (2010)
TD and RA1 were supposedly released to mark the 12th and 13th anniversaries as well, but I don't believe that, yeah right.
Red Alert 2 was actually 'free' as bonus disks with Red Alert 3 (only pre-ordered copies I think, I've forgotten now)
So, going by all that, I'm guessing that the next C&C game to be released as freeware will be released when (or if
) EA decides to develop another C&C product. Either that or when some rich dude decides to buy the rights from EA