There are major advantages to the DOT baggage fee rules, but this is an example of an unintended consequence and a fight that neither DOT nor the carriers want. As it involves not only US carriers, but any ticket to/from or through the US, it also involves broad swaths of any non-US global carrier with a substantial US business.

The problem is that there is a substantial possibility that DOT would take the position that if a carrier treats cross-ticket features as though on a single ticket, that the DOT bag fee rules ought to apply. But, non-*A carriers can't see what the first segment was when the passenger returns. Thus, the pax goes to check-in on the return, the passenger goes off the rails, complains to DOT and there is not only a poor customer experience, but the regulatory costs of dealing with DOT.