I don't see how liability figures into this, properly trained people should have access to the repair documentation. But as we can see they have insecurity issues since they insist on potting circuits that don't require potting and they rub part numbers off parts. I would opt for believing the design is crap and they are saving face through obscurity.
I don't buy into the legal liability argument one bit.
I agree that obfuscation is definitely part of the equation, especially as demonstrated in Shahriar's teardown. However, in the end they are selling the device to operators, not trained service technicians under contract / employment. Every test equipment manufacturer I can think of advises end users as much as possible to not disassemble the equipment and leave servicing up to trained repair personnel. I doubt those cautionary statements are made solely as a matter of politeness. Liability potentially arises if they provide you with the information you need to get yourself into trouble. As sad as that may sound, that is the current legal climate.
How much do they gain and how much do they risk by providing a 2nd hand end user with their documentation? Where do the incentives lie? When companies are willing to provide customers with detailed service manuals and schematics, I think they are going out of their way to do their customers a (much appreciated) favor.