The United States had 12 capital ships of 40,000 tons under construction when the treaty was signed. The only country that didn't need the treaty was the US. If the British and/or Japanese had tried to start a naval arms race, the interventionist and isolationist factions in Congress would have happily come together to fund any amount of construction necessary to match foreign navies.Where there's a will, there's a way. We know that OTL the US was actively spying (bugging) the Japanese and British deligates hotel rooms and offices. If that was found out the UK could basically go "Agree to this or we tell japan and this all falls apart."