It has been about 30 years since I was involved with one of these walkways in the Twin Cities.
There is an agreement between the City and the building owner. In the building, it is owned by the property owner with a public easement for the use of the general public. Outside of the building it is owned and maintained by the City. The building owner gets people into his store that would not normally go there if it were not connected to the skywalk, particularly in the winter.
However, sometimes, what is in the easement and what is NOT in the easement is unclear.