This may mean they won't honor a warranty on a non-DOS system at all, or it may mean they'll insist on installing the Unix on disk themselves. This means you have no options if a part doesn't work. Avoid any company with this policy. Possibly much longer. Knowing the restocking fee can be particularly important, as they make keep you from getting real satisfaction on a bad major part.
Warranties are tricky.
There are companies whose warranties are invalidated by opening the case. Some of those companies sell upgradeable systems, but only authorized service centers can do upgrades without invalidating the warranty. Sometimes a system is purchased with the warranty already invalidated. If the vendor is not an authorized service center, the manufacturer's warranty is invalidated. The only recourse in case of a problem is the vendor's warranty.