I know some people can damage their knives by taking it apart.
However, I still don't know why taking apart and cleaning your knife(properly with no damage) voids warranty.
I take my knives apart all the time.
However, if you took the knife apart and destroyed it or messed up something, I could see them not fixing it because after all, it was your fault, not theirs.
But if I take my knife apart and clean it, and then a problem comes up that is a manufacturing issue or something of the like, I see no reason why my taking the knife apart should void getting an issue fixed.
As an example, I'll use this post.
Some people have a mechanical inclination and others don't. Production and mid tech knives are assembled by technicians with years of experience working with small parts, precision equipment, and a high level of skill. This can be said for techs from Kershaw to Chris Reeve, to Benchmade, to Spyderco, whatever.
Sal once stated (more or less) that a knife maker puts your knife together. When you, the end user, a "non-knife maker" disassembles it and reassembles it; you are in essence disturbing the craftsmanship aspect of the knife itself. I'm paraphrasing, as I don't have the urge to search for his post on the matter, but in so many words that is what was said.
As a "non-knife maker/technician" what makes you think that your knife requires complete disassembly to clean? Yes, it is technically "your" knife and property, but relying on someone else to fix it, for free no less can be absurd in my view. In my line of work, I've seen what people try and do to their networks and servers by claiming to do "basic maintenance". They expect us to walk in and restore data, reconfigure devices with complex configurations to begin with for free, because the devices have a warranty. The warranty doesn't curiosity induced disaster. A simple mistake can create hours of work.
The more time a knife maker has to spend fixing someone's mistake, then that is less time they are spending producing new pieces for the company. It's like a hole in your pocket with cash hemmoraging out. Knifemakers cannot make a broad assumption that everyone that stumbles upon their products has the tools and cognitive ability to tear down and reassemble the knife and have it be as it was from the factory. I've also seen people disassemble $900-$1500 semi custom and custom handguns, only to not be able to reassemble them again. Very dangerous in that case for what I hope are obvious reasons.