in 2002 to 2004, when tier 1 and then tier 2 Carb regulations were first implemented for "large spark ignited engines, above 25 horse power", (which is how the regulation addresses forklift motors), I know the Technical communicators and trainers working for MCFA dealers addressed this same subject and concern with MCFA service engineering management.
There was no real clear directions from MCFA, at that time, that said any thing like "only dealer techs can have this software", only that the reasoning had to do with the C.A.R.B. regulation's definitions of "tamper proof" and the regulations requirements for the manufacturers to only allow properly trained and certified as competent people to repair and replace components, to insure compliance with specifacions for the required time frames and that the components would be -correct- fit, not "will fit".
It is my understanding that since that point, other manufacturers, or their dealer principals, had defined the regulation considerably more narrowly, and so other dealers had followed suit, with an abundance of concern to correctly follow the regulation and "industry standards".
I am also pretty sure that the pricing from the OEMs for the required training, software and cabling is not so much "what the market will bear", as it is a normal % markup from what the vendors charge, in other words, what it costs the OEM to provide. Consider how much Microsoft gets for a DVD. The DVD and packaging may only cost 4 US dollars, but they justify $400.00+ for "Office" applications based on the expense of the creation of that same software, and they are selling hundreds of thousands, not hundreds like a forklift OEM.
I would also like to make a note that not all manufacturers have the same rules, but I know that MCFA had a design requirement that only a very minimum amount of items require the software (etc.) for repair, only those items that would effect the safety or emissions components require the software, even the fault codes can be retrieved and most of them cleared without the software, that the software (etc.) does not really "fix" very much. having the software and cabling does not replace good understanding of the systems, and by no mean is a panacea.
Just because it has a place to hook up a computer does not mean that only with a computer or even that you need a computer for you to fix the forklift.
I would also note that this problem (dealers not willing to sell compition cables to access the controls of the truck) did NOT start with CARB, and had existed in some brands of electric forklifts for years before 2003, when tier 1 carb became effective