Americium can’t be used to build a nuclear bomb. I think it’s entirely reasonable for a country to overreact to nuclear arms control, especially if there are escape hatches like the one used in this case to let people off the hook when deserved.
It's never reasonable to overreact.
Regular old garden variety proportional response should suffice.
Only plutonium 239 can be used to make nukes. Assume it was plutonium 238 that this person bough. Same thing goes with uranium. Why you're allowed to buy it, because you can't turn it into a bomb.