Unlike any other. Agreed.
Now, nobody committed to buy their declared numbers at the start but they had to start somewhere, so an initial statement of purchase numbers was made by all partners, to allocate the industrial work share. You're right, there was no contract per se, just mutual trust and an agreement on numbers between the US DoD and each country's Armed Forces and lo, the work share was allocated. Canada declared 65 IIRC.
Extricating yourself from the Program doesn't give you Grandfather rights to continue to make a select group widgets and components for all F-35s for the next 40-50 years.