May be caused by the difference between hardware and software support.
Most computer maufacturers will have different rules for supporting software and hardware.
I believe that Evesham will offer support for the software originally supplied with a machine, but not for anythng else which is added later. They would probably argue that the support for the new software should come from whoever sold that new piece of software.
Of course for many users, especially when it comes to operating systems, this seems a strange distinction.
I can see the manufacturers point, if one installs something bought elsewhere why should they bear the cost of supporting that item? At the same time upgrading is a fact of life for many computer users and like osben they may well feel they should have support from someone who has taken a large amount of money from them.
The truth is that is hard to argue that a manufacturer has even a moral responsibility, much less a legal one, to support software they haven't supplied and, whilst a more flexible approach would be appreciated by many people, if as a result people with undeniable entitlement to support couldn't get through there would be many complaints