That was my memory that in 2001 or 2002 BIL lost it's international status. It does not have international status now so when it was lost is irrelevant at this point (BIL now listed as municipal). The loss of customs standing means that no international flights can land intentionally at BIL without landing at another US airport first. This obviously excludes the incoming emergency landing at BIL but pax might not be allowed to de-plane as they cannot offically clear customs at BIL. The friendly skies agreement may state that passengers may file customs forms ect. on the plane and not go through the formal customs line of passports, ect. However, customs officials must be present at the airport for any special cases that may arise from the international flight (firearms declaration or others).

I am a bit confused as your last sentence noted having to land in BIL to go through customs. A YYC-DEN would not land have to land in BIL even if BIL was international as DEN is an international airport. The Customs process officially occurs at the landing point, not upon entering the airspace. Think about the hassles of flying to FRA if this were the case having to file Irish, French and German custom forms prior to EU even though you never landed in Ireland or France.