Perhaps BG and SW have different policies? It seems like a entirely different ticket if you wish to get 2 days from BG.
All I can tell you is that I called seaworld and asked if I could get 20% off the ticket(s) w/ SW rapid rewards card, after the response I asked if I could use this discount and get the second day free benefit, and was assured that I could. When I visited the park it was exactly as promised, no questions about it, no hassles. If they had told me I couldn't get the second day free as it was discounted, I was prepared to get speak with the GM of the park if need be, as I was told by phone, prior to deciding to visit the park, that I would get the second day free....
So even though they are both Bush parks BG and SW may have different policies. Another difference I see in the return a second day for free is that SW second day must be within 7 days of the first, BG is by the end of the year.
I am sure that if you really wanted the second day free, and that is a slightly higher priced ticket they would allow you to upgrade, prior to visiting the park.