+1 to Vidia-king. This is a standard industry policy that makes perfect sense. Greed has nothing to do with it - how many people are gonna try to RMA a machine that they installed Linux on? It's an insignificant amount that has nothing to do with their profits.
Trying to isolate hardware vs software issues and spending hours on fixing one problem is inefficient and is frankly not their problem.
A poster named "Consumed" in the original link nails the issue on the head:
I worked in a retail computer sales shop awhile back and HATED when customers returned a laptop with Linux on it. Made troubleshooting a pain in the ass because some models of computers had special drivers or software that made the unit work properly only in Windows, and 99% of these computers don't come with OS recovery discs anymore. Yes you can burn your own recovery discs within Windows but most consumers don't know this is possible.
So technically while it's highly unlikely that Linux would cause hardware damage or alteration, it sometimes throws the hardware in a loop due to thermal controls, etc only working from within the operating system. Yes very poor design, this stuff should be built into the BIOS/EFI, but some models of laptops are junk and they depend on proper OS/drivers to work properly. Very rare, but it happens.
This is a very gray area which I wish retailers would take a more clear stance on. Removing the factory operating system and putting Linux makes things more difficult for everyone involved, and increases the costs all around. Please, if you're going to return a laptop, at least put Windows back on it first. Thanks.