I can see why they did it. They want people buying the DLC and using the new raid weapons. If they allowed everyone to upgrade the VoG weapons to 331, there would be no need to strive for the Crota weapons at all. I'm expecting (hoping) that when the next DLC comes out, VoG weapons will be upgraded to 331. New House of Wolves weapons will be higher attack so people have a reason to grind for them, but it'd be silly to leave the VoG weapons at 300 as there would be no reason to play through a major part of the game if they're 2 levels below.