The general headcanon is that rent, access to the stations grid and crew for maintenance is credited at fair market value for officers to use at Quarks. Plus having a steady stream of Starfleet or Bajoran officers there would deter general crime and ruckus, much like why gas stations tend to offer free coffee for cops.
This question came up a lot in DaystromInstitute.
The general headcanon is that rent, access to the stations grid and crew for maintenance is credited at fair market value for officers to use at Quarks. Plus having a steady stream of Starfleet or Bajoran officers there would deter general crime and ruckus, much like why gas stations tend to offer free coffee for cops.