Despite everybody's best efforts, none of today's PC operating systems actually work properly on a brand new PC. In say, Fedora this can be painfully obvious as your brand new HP laptop runs X with VESA graphics or refuses to acknowledge the existence of its built-in touchpad. Sure there'll be an update that fixes it, but that's later, and you've got the new laptop now.
In Windows this is hidden by pre-installing the necessary patches, new drivers, utilities etc. at the factory.
But if the user re-installs from a "clean" OS DVD they don't get any of that, and then end up with the experience familiar to bleeding edge Linux people. They will, of course, report this as a fault, which will cost someone a bunch of support money. Hence, there is no way to install a clean OS, because they know it won't work properly anyway.
You mention trademark law as a solution to the dual boot problem. This is charmingly naive - to sophisticates like us it's obvious if you're running OS/2 or Mac OS X, but to the average user it's all a muddle. My sister continues to be caught out by the need to buy different software for her "new computer" (a MacBook). Trademarks don't help because they're not aware of the trademarks, it's just more technical mumbo-jumbo.
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