First, the idea is to give America an incentive not to let England, Africa and Europe fall to Germany unopposed. Moving the Objective to these territories, effectively, does that. But since it is an incentive, America does not HAVE to move to stop it.
Second, since the NO would only start on Round 4 (generally) it gives the Axis time to stop it.
Third, if America invests in the Atlantic to keep the NO, it loses equipment against Japan - YAY we are helping Japan, the only country in this game that needs help!
Fourth, if America decides not to invest in the Atlantic for the NO, it loses 5 IPC a round it can use against Japan - YAY we are helping Japan, the only country in this game that needs help!
Fifth, if America does invest in the Atlantic only, then Japan will win the game - YAY we effectively split the American build and made them just like every other country, able to put only 50 IPC on any given side of the board at a time!