Aren't they, potentially, disenfranchising their own voters?
Yes, and that prospect is mentioned in the article...it describes a situation where MA voters go in a majority for one side, but the other side wins the national vote (A very possible outcome in 2012, with MA staying pure moonbat while an actual non-Communist wins the national vote). This law would essentially substitute the vote of non-MA voters for actual MA voters and be subject to attack for that reason.
On top of that, as many mentioned above, this is never going to get to the 270-electoral-vote buy-in level necessary to trigger it.