These plates are a sign of the president's commitment to increased (i.e. voting) representation in Congress for DC. There are opposing arguments on this issue, however there is no license plate available to a private citizen of DC to express an opposing opinion.
In a recent federal court decision Judge James Fox concluded that "the State's (NC) offering of a Choose Life license in the absence of a pro-choice plate constitutes viewpoint discrimination in violation of the First Amendment." and the plates were permanently banned.
There are enough similarities between the two situations to believe that the same reasoning could be applied to the DC plates finding them to be unconstitutional, and thus that the president is embracing and sponsoring, an avenue of free speech denied to some.