Should it be illegal for a QB who slows down towards the sideline to come back and run?

[Small Town Values]

Yes! Because it's sneaky, & all sneaky stuff should be banned. 

[Bobikus]

If he's doing this past the LoS when he's definitely considered a runner, then it's just a good play.  It's a bit sketchier when a QB does this down the sidelines starting behind the LoS or running alongside it.

[BayRaider]

Definitely not. A defender has to play the ball until he is fully out of bounds. If a QB sees a defender not finishing the play out that's the defenders own fault.