I think Busch Gardens and Sea World have figured out that if they charge a lot of money for multi-day visits, most people will just go someplace they haven't been before . . . the parks don't make as much as if they allow guests to get back into their parks "for free" during a vacation, where presumably they spend a lot of money on food/games/souvenirs etc. during their repeat visit.
We just got back from SeaWorld and purchased the "fun card" which allows us access to SW and Busch Gardens for the rest of the year, for less than the cost of two one-day tickets. We live less than an hour from BG, and we'll probably take the family up several times on weekend days when we have nothing else going on. We don't have to worry about admission, but you can bet Busch Gardens will make some serious money on us when you factor in parking, lunch, arcade games, t-shirts, etc. Works out great for everyone.