Reserves are like Durations: they wouldn't function any differently if they didn't have their own unique type, but they have their own unique type because they have something special about them that sets them aside from every other action card, and giving them a different color just helps us remember that it has that unique effect. Sure, the text says it all for us, too, but that's also true of Durations. There's no harm in having the unique color (the different color isn't likely to confuse anybody, I don't think), and it looks cool, so I don't see any problem.
As for why Distant Lands is a Reserve card, I agree that it probably didn't need to be, but that also would have made it the only card in the set that could go on your Tavern mat that wasn't a Reserve card. True, Coppers can also do that, but it obviously wan't realistic to go back and turn Coppers into Reserves for the sake of one kingdom card that's going to come up roughly once in every 23.6 games. Overall I just think there's less confusion if Distant Lands in a Reserve.