From what I understand the reason they made replays 30fps was to make them seem more realistic!
When you watch anything on TV it will have been recorded at 25fps for UK/PAL and 29.67fps (or something like that) for US/NTSC!
The reason it makes it more realistic is little things like when the wheels are spinning 30 times a second they should appear to be standing still when you watch racing, etc on TV, but if the game did replays at 60fps, then it would only do that when the wheels were spinning 60 times per second.
So I think its just to make it seem like your watching the replay on TV, but the fast gameplay obviously makes use of the 60fps.