At some point this isn't a subjective matter. The reason being that the defender IS allowed to make a certain amount of contact. But where there is a give, there is also allowed a take. A WR shouldn't have to break from his sprint when a Corner gives him a push that is within his defensive limits. And that's regardless of whether or not a sideline is in the context of the play. If a Corner wants to box in WR on a sideline route he can't give the guy a shove. The moment he does he extends the playing field of that WR a tad.