That may be a violation of Hilton brand standards (if the property is a designated Airport Hilton):

4.1 AIRPORT PRODUCT LINE
4.1.1. Airport product line designation must be approved by the Brand.
4.1.2. The hotel must be located within a three-mile / 4.8 kilometer radius of the entrance to a commercial airport to have the airport product line designation.
4.1.3. If the hotel is designated as an “Airport Hotel”, and unless the hotel is directly adjacent to and within walking distance of the airport, it must provide a complimentary airport van/transport service so long as local transport regulations permit such a service.
4.1.4. If the hotel is designated as an “Airport Hotel”, guests must not wait longer than 15 minutes for transportation to and from the airport or hotel between the hours of 6:00 a.m. and 9:00 p.m., and no longer than 20 minutes at any other time.