No McMaster Carr is just being really anal about ITAR that's all. They probably don't want to pay the fees and maintain the paperwork to certify what they are exporting is not part of a weapon system, can't be used as part of a weapon system, or is not of technological value to create a weapon system or enhance a weapon system.
Technically every screw under the sun could be used as a fastener on a weapon system of some sort somewhere. But not every screw will land up on such a thing. But McM/C is a supplier of industrial products so Uncle Sam scrutinizes them closely. If they were a listed as a motorcycle parts wholesale warehouse Uncle Sam wouldn't blink an eye at who they sold what too. Until something went terribly awry and that bolt for the 1968 350 Wazzit was found to be a suitable replacement part for an Iranian WMD. Then things would get ugly at light speed. Can't blame them really, nobody wants to hang out at club fed for the next 20 or so yrs.