Often when DL does something that is only out of Europe, it is to harmonize with AFKL so there is not a particular benefit to booking through a specific airline. My guess in this case is that AFKL fares don’t book into C+ domestically and DL (intentionally or unintentionally) matched that.
Yes, they harmonize fare filings between US and EU as part of JV (but not necessarily bucket inventory availability on individual flights). You can see this below on roundtrip PE fares from AMS to DTW (AFKL uses different booking classes for PE than DL does which is why the booking classes are different while pricing is the same). Unlike domestic US fares which are symmetric, ex-US and eu-EU fares are not symmetric and will have separate fare filings, pricing, and rules. ex-EU fares will apply with either a one-way or a roundtrip booking ex-EU (the same for ex-US). Yes, AFKL doesn't have a booking class for C+ (only VS does -- Economy Delight), but that doesn't really explain why non-refundable PE fares book into Y on US domestic, while refundable PE fares book into a DL C+ W class on US domestic. But since the fare filings are asymmetric, it's apparently a decision they've made.
