They are at "Canada" customs... there are no restrictions on importing sights or optics... so you are fine. However, there are restrictions on EXPORTING sights and optics from the USA, unless the OEM has completed the applicable ITAR paperwork... many companies don't complete beacuse;
1. They are small and can't absorb the cost.
2. They don't see Canada as a viable market.
3. Couldn't be bothered with the hassle.
Large US based retailers, such as Brownell's and Cabela's, generally keep track of which products are approved for export... they don't post on the product page, but often if you add an item to your shopping cart, an alert will come notifying that this particular item "may not be exported."
When this occurs, either the manufacturer has not completed the necessary paperwork, or the item is listed and stipulated as non-exportable by ITAR... as was the case with Boyd's formerly named "Tacticool" stocks.