Isuzu service is pathetic

Pathetic service. Bought a second-hand Isuzu bakkie from dealership in Johannesburg (great service from them when we bought the bakkie) we were told that included in what we paid is the 90000km service is included at any Isuzu dealership because we are from Underbergin kzn. Wanted to book a service now during the June school holidays(I am a teacher and it's our holiday now, our closest dealership is 150km away and it takes a whole day for that) so tried to book at Key Isuzu in Pietermaritzburg and they said because the bakkie is over 90 000km and when we bought it it was done on 73000km and not on 75000km so according to the warranty and policy they can't service it because it has gone over the allowed 1500km so now we have to pay R5800 for the service. My question is if you already paid for it when purchasing the vehicle, why can't they still do the service? It would be different if I wanted to claim for something and they don't want to pay for it because it was not serviced and that caused the damage, but to not want to do a service that was already paid for is **************** and pathetic from Isuzu. There is nothing wrongwiththevehiclejust wanted the service to be done, but fir some reasonit can only be done if you pay for it AGAIN . Can you imagine how much money they ***** when they do this to a few hundred cars a year. Really disappointed, angry and frustrated.