Uhh... unless I'm missing something, you're not even flying Delta... and yes, many European airlines charge seat reservation fees. Air France, KLM, Virgin Atlantic, British Airways, Lufthansa...
And I bet that if you had clicked on the seat assignment link during booking, you were presented with language like this:
This flight is operated by our partner, KLM, and the seat map for this flight is not available. After purchasing your flight, select your seat by visiting My Trips. Fees for seat selection may be applicable based on class of service, frequent flyer status, or days to departure.