Whats to stop them from giving out proprietary information?
Well... nothing really, anyone that has it can pass it on if they are willing to take that chance.
The only deterrent in place at the moment is the DMCA (Digital Millenium Copyright Act), or in software they do have methods in place to bind the programs to the computer they are installed on which makes the software virtually bound to the device and it cannot be transferred to any other by any conventional means.

In any case though If the owners of the information wished to do so they could make someones life VERY miserable and financially difficult if they wished to do so if they felt these terms were being violated enough to warrant such an action. The deterrent in place would make anyone think twice IF they really took it seriously.

Even on this forum we walk a thin line by sharing what we know when it comes to diagnosing problems and sharing our knowledge gained from our training on how to repair a lift. But i believe the one thing that would put anyone over the line would be the blatant disregard for this law and passing the material such as manuals around like they were nothing more than free information when in fact the opposite is true in this case. On every site i use to get my information from there is always a warning informing the users of the site about the information obtained from there stating this fact.
If the manufacturers or dealerships decided they were loosing business or financial gain in a substantial enough manner from these actions they could by rights prosecute anyone violating these principles. These laws seem to be more prevalent in the United States but i have seen some cases where it reached out further beyond those boundaries.

A similar example of this is in the satellite industry with all the signal hacking that was going on years ago and thousands of end users hacking the systems to gain access to their proprietary property in order to gain free "paid programming". This information was obtained illegally and passed around so much these mainstream companies were loosing so much money they had to do something about it. The usual deterrents were put in place but were circumvented so quickly the only recourse for the satellite companies was to go after the hackers and the end users in the end, that pretty much put a stop to it. They gained enough time to design a system that implemented a new set of security rules that made it quite a bit more difficult to circumvent and that combined with putting the fear of financial ruin and jail time into the picture nobody wants to attempt it on any large scale or much less make it public.

Now i know in the lift truck industry it is not as wide spread as the above example and any amount of money that may be lost here is miniscule compared to that example but its the same principle that applies in any case.

From one tech to another though, we all need help from time to time because of the growing technology in this industry and sharing information is what gets us to our end goals , "making sure the customers needs are met" but there are proper ways of obtaining these goals and we all should follow them imo. I myself will help anyone that asks for it, and send them down the right path so they can obtain their goal of repairing their machine but i do have my limitations on how far i will go with that.

I could make a novel about my thoughts on all of this but I've already said enough and i'll leave it at this for now.