I know there exist range of everyday goods that can be used in order to make weapons and they are more tightly controlled than other goods, especially in import/export.
Does a (possibly) high quality lens fits into that? I don't know, but it rang a bell immediately I heard about the restriction.
Which...wouldn't be an issue, if, upon receiving your camera for service, they didn't refuse to work on a camera that wasn't in its proper market.
Note I didn't mention warranty service/repair. For decades, they wouldn't even touch a Japanese-market product at the US service center, whether you were paying or not.
At the time, Canon USA would happily warranty or otherwise service any Canon gear you sent to them, regardless of where it was bought.
Vacation in Japan, decide you want to buy a lens for your US Nikon, or maybe your US Nikon lens is lost/stolen/breaks and you get a replacement...years go by, it stops working so you send it to nikon for service...and promptly are told to get fucked, and it's shipped back to you.
I hear Canon is now pulling similar nonsense. I know someone who dropped several thousand dollars on a Canon mirrorless camera body at Abes Of Maine, only to find out (when the camera almost immediately had an issue) that the camera was a grey-market import and Canon told them to get fucked when they called for tech support. Abes Of Maine ghosted them until they threatened to do a chargeback ("goods delivered not as described", I believe.)
Oh, also: Nikon were dicks about their warranties, too. Non-transferable and they required proof of purchase, so you couldn't even buy an unregistered camera from someone else and then register it yourself.
Canon USA? At the time, they only cared if the "clock" hadn't run out, not who bought it.
All this is why I bought into the Canon ecosystem multiple times. Nikon is never getting a dime from me.