That cart has an "anti-cheat" feature- both joysticks have to be pressed in order to start the game. That way the pitcher can't throw a bunch of strikes before the batter is ready. With normal key mapping, hit CTRL and A to start.
We did just find a bug in the decimal addition opcodes of the F8 emulation core, but I just checked and it doesn't affect starting Baseball. It might affect scoring, though.