Good point. Every time you see passengers flying from BWI to CLT so they can connect to Europe, that's some evidence of the weakness of the pmUS network and specifically, the weakness of CLT to Europe. Logically, BWI passengers should be flying to PHL to connect to Europe, right?
BWI-CLT is 361 miles in the opposite direction from the CLT-Europe flights - that's a lot of wasted backtracking. BWI isn't going to have flights to Europe (other than BA's LHR) so everyone flying from BWI will be connecting. In the future, they should probably connect at PHL or JFK, helping to top-off flights that already have better O&D to Europe than Charlotte offers.