Originally Posted by
Often1
The answer is simple. OP accepted a voluntary reroute. He is subject to the fare rules, including bag fees of the new carrier and to a refund of the fare difference (which here is apparently $0).
Neither is DL obligated to accept the bag allowance from the US ticket nor is DL obligated to refund OP anything for having paid fees to DL.
OP accepted the voluntary reroute based, in part, on Delta's promise about OP getting 3 free bags.