Might as well trademark Xerox or Kleenex.
By now, DWG is used commonly in the vernacular that it has lost all possibilities of a trademark. Along those lines, similar to Xerox and Kleenex, DWG has become a household name so even if they did originally trademark it, that trademark would now be lost.
Their lawyers must be either be bored and this gives them a task to perform, however worthless. Or they actually expect to be able to enforce with trademark to earn some quick cash through litigation settlements or back-paid royalties.