Sure, forwards will also be sneaky to take the mark, but generally their aim is to mark the ball, the defender can spoil the ball and 'win' the contest. Shepherding, bumping etc. aren't usually allowed in marking contests (bumping to some extent), and if you aren't focussed on the ball the umpire will penalise you.So a defender will "do anything" to stop the mark being taken but a forward won't "do anything" to take the mark? Nah. Also, the man is allowed to be the focus, afterall footballers shepherd, bump, hold, etc. I'm not saying Rance doesn't get away with a lot of scragging, which isn't entirely against the rules, but forwards aren't all non-contact types either.
Forwards perhaps get the benefit of the doubt and if they appear to be getting prevented from contesting the ball get the free. Not sure about Rance in particular, just speaking generally.