As I pointed out these "vendors" are breaking laws in several countries. As far as a freedom of speach issue goes you are correct, nobody can fault you for linking to another on the web... but by highlighting these vendors so prominantly on YOUR site might likely be construed as "endorsement". That could get you in a pile of hot water given several are criminal enterprises. (I'm talking about the seed dealers and the weed deliverer, not RiverRock).
RiverRock is an illegal business in the United States, same as all other Colorado marijuana businesses which lack permission from federal authorities. Tomorrow morning, the US drug enforcement agency can go to Colorado and Washington and start making arrests and seizures. Only thing stopping them is in the current man in the white house. Banks won't openly accept marijuana businesses because they know such businesses are currently illegal.