I don't understand if your question is about GSX or KLAX airport. If with "service" you mean the GSX ground vehicles, then this is really a question that is relevant to GSX, and shouldn't be posted here.
In any case, I think you are confusing what airlines are allowed to a Terminal, which is something related to the scenery AFCAD, with which service the airplane gets, which is related to the GSX vehicle config.
First, as explained in the manual and many times already in the forum, what airline *you* are flying doesn't have anything to do with the service vehicles. The service vehicles are chosen according to the airport and the atc_parking_codes of the *parking* in the scenery AFCAD.
This means, while Air Canada has a preference set for AI airplanes at KLAX, so you should see Air Canada AI airplanes going to their preferred parkings, it's not configured in GSX as a servicing company for KLAX, so it's just normal you won't see Air Canada vehicles servicing at KLAX.
