I tried to refrain myself from participating in this discussion in fear of being misunderstood. Let me proceed to say that I have spent more $$ on Spyderco knives than any other brand and still possess more Spyderco knives than any other brand. With that being said, it appears to me that many other manufacturers have survived and succeeded by allowing customers to take apart their knives and even giving out free screws and washers and on top of that, they do not necessarily set their knife prices significantly higher than Spyderco.
For the folks that have several hundred folders but managed to have never taken apart any of them, I truly admire you. However, for me and I suspect for many others as well, part of the joy of this hobby does involve tinkering. Folding knives are tools (and toys), and tools in particular simpler tools (which folding knives fall into) are supposed to be maintainable, most often by customers. Are there people who are not good at or even cannot take apart and put back properly a folding knife? Likely. But I'd venture to believe there are much much more who can do so properly. Shall a manufacture discourage or even disallow customers from taking apart the knives just because of a small percentage of incapable customers? I don't know.
Even with improperly disassembling a folding knife, what kind of damage can be caused on the knife? (The guy may cut himself badly but I don't think that is the kind of issue we are discussing here). Most likely if not all, it is just some stripped screws, the cost of which compared to the cost of most folding knives nowadays, is really little. We don't see the blade got broken, the handle shattered, the lockface ruined, etc. because of disassembly do we? And even if they did, nobody expected the warranty would cover it (hence, beyond of the scope of discussion here).