I think some further explanation might be warranted here. The lift truck manufacturer has every right to designate who performs WARRANTY WORK. Most, if not all warranties specifically state that WARRANTY REPAIRS MUST BE performed by an Authorized Dealer. This is the normal process, just as a car or television or cell phone, etc. The manufacturer is paying the bill; they get to choose who they pay for that. That makes sense. The dealer that represents the manufacturer is obligated by contract, (law) to perform warranty repairs that fit within the definition of warranty.
Likewise, the lift truck owner has every right to choose who performs routine repairs, maintenance and other non-warranty repairs, assuming that those repairs are performed by someone qualified to make that repair. And, the scheduled service is performed on time and properly documented.
As Mr. bbforks and others pointed out, it is in the lift truck owners best interest to have the dealer perform any maintenance, other than very minor non-warranty repairs, during the warranty period. The dealer has access to training and service information that others may not have such as, recalls, service bulletins and proper repair techniques. Having a dealer do all the work during the warranty period helps to ensure that small problems don't become big problems later on, as they are in the best position to know how to solve them early on.
As a lift truck owner, if you are not satisfied with the dealer response then by all means, contact the manufacturer and work it out BEFORE it costs you money!