Actually based on this it seems that the system isn't random. People that are visiting a lot aren't getting perks as they don't seem to need an incentive to come visit the park. Those coming only once or twice, the park is trying to get to come more frequently so that they will spend money on food, drinks, games, etc. while in the park. And in the case of the perk Stoan showed above, they are doing it by offering a fantastic perk to bring 4 additional people with you so they can collect as much money in that one extra visit they are getting out of you with the perk.
As Tr0y said, it would be nice if rewards were based on in park spending so they more you go/spend the more you get, but it seems likely it is the reverse and it is just being used to entice people back that aren't coming as frequently.