Nothing forces the US to focus their pre war production on Japan in the Global War campaign. In fact, at this year's eurowifcon, the US player at my table did quite the opposite, and focused most of his builds vs Germany (me), which was quite a lot, since he took option 34 in M/J 41, and was at total war in S/O 41.
Even with the great production boost that he enjoyed, it wasn't until late 1942 that he had all available Essex class carriers on the production circle, and most of his BB were in the Med.
Btw, he did take CW Reinforce the Pacific before option 34, to make sure that they could garrison Rabaul. Together with quite heavy sub focus, this enabled him to still put some pressure on Japan in the early war.