No, that's not the rule but often happens that a client flies an escort for an overnight and instead of going back home the next day he decides to stay in town a few days seeing other clients.
I know escorts who won't fly anywhere without an "anchor", a client who will pay for the flight and/or commit to multiple hours to see him guaranteeing a profitable trip.
Maybe the guy you want is concerned about investing money on a flight and hotel and have clients flake on him making him loose money. Possibly he's been burned before going to places to have guys cancel because of a death in the family, flu, falling in love, etc.
At the end it's up to you to make the choice to invite him or not.