Leaving the second BB alone is bad for two reasons:
You gave England a capital warship that can bombard you at will. That means two bombardments for the price of 1 infantry. (6 IPC for 3? That’s a good trade!)
England is not out 16 IPC for a carrier to defender herself.
Don’t forget that the Allies don’t have to worry about keeping 1-2 Figs on the carrier giving them even more versatility
If all 4 units to be moved are still in EUS at the start of the US move, then the Algeria amphib can be done using only 1 TRN and the 2 units it can carry, the other INF and the AA would have to be moved in NCM (assuming the US won the amphib)
Just think of it this way: Since trannies are NOT involved in the attack (they are not even on the battle board) then they don’t have to “waste any part of their move” getting to the objective. However, any BBs or DDs that contributed to a battle offshore canNOT bombard during the amphibious assault.