Most public airports are built and maintained in part by federal grant money as part of the interstate air commerce infrastructure. This means that if the airport provides a service such as fuel, it needs to provide that for every potential aircraft that can use the facility. In other words, the grant money depends on providing both JET-A and 100LL. If they don’t do this then the FAA can force the airport to refund grant money and impose a fine.
This assumes, of course, that the FSDO has the spine to go after the airport for doing this, which most don’t seem to have lately.