There's a subsection of griefing that entails blocking exits/entrances and pathways. Setting up toll stations of the like might seem innocent at 1st, but honestly if some random person in real life said to me "50 bucks or your dead buddy!", he'd be dead fast. I guess that's why the rule on blocking transitions is in effect. Threatening people in any form in echange for their property... you have to EXPECT violence as a rule. I'd have to say that when non-violent crimes like pickpocketing are disallowed, other forms of theft like muggings and shakedowns would be as well, and even moreso. One has far more moral leeway hacking the head off a bandit than hacking the hand off a begger.
Besides, even if it was allowed, too many people would only mug folk lower level than themselves, and suddenly become meta-nice when they see the purple "Impossible" text above their descriptions. Suddenly when Bjorn and Mith cross the bridge, there's just a guy standing there, minding his own buisiness... But woe becomes anybody with green text! I'm not saying that you'd do this... just that if you set up toll stations, about 20 other people and their grandmothers would be eagerly doing this within 10 minutes.