Milwaukee is an accommodated city, but not a gateway city. Hence the additional fee.
From https://www.rssc.com/experience/customize-your-journey/air-program :
*FREE Roundtrip Air includes ground transfers between airport and ship and applies to coach, roundtrip flights only from the following airports: ATL, BOS, CLT, DEN, DFW, DTW, EWR, FLL, IAD, IAH, JFK, LAX, LGA, MCO, MDW, MIA, MSP, ORD, PBI, PHL, PHX, SAN, SAV, SEA, SFO, TPA, YUL, YVR, YYZ. Airfare is available from select other U.S. and Canadian gateways for an additional charge. Airline fees, surcharges and government taxes are included, however, airline-imposed personal charges such as baggage fees may apply. FREE Business Class Air applies to intercontinental flights only. Custom Air arrangements may be made 270 days prior to sailing (custom air fees apply). FREE First Class Air applies to all domestic (U.S.) ports for guests sailing in the Regent Suite on Seven Seas Explorer®. Hospitality Suite access and Disembarkation Day Tour are offered in select ports only provided minimum guest requirement is met. Dayrooms are capacity-controlled and may not be available within 60 days of departure.