Considering I'm not aware of any other hotels in MKE or anywhere else in WI charging this fee, yes I think the fee would not be there if it wasn't Bonvoy. I've stayed in numerous other hotels in MKE city, MKE county and the state of WI and never been charged a "sustainability fee." I also don't stay at Bonvoy hotels on my own dime. So either Hilton and Hyatt are eating the cost of this "fee," or Bonvoy wants to look cheaper on OTAs by having a room rate that is $15 less per night. Which of those is more likely?