Same is true of the installation manuals for electric receptacles, light fixtures, dishwashers, ovens, garbage disposals, water heaters, furnaces..... Yet it's perfectly legal here in most parts of the US for a homeowner to install those themselves.
Vehicle factory workshop
I've seen both factory manuals and aftermarket ones, eg Chiltons, Bentley and I don't see a lot of difference. I thought the main purpose of the aftermarket ones was to offer something to the DIYer when the factory ones either may not be available at all to a DIY or the aftermarket ones are a lot cheaper.
Then you have a crooked insurance company and you should go straight to the regulators. You car catches fire in the garage, burns the house down and they can deny the claim because someone unlicensed put in a receptacle in the bathroom?
People have made the claim that work you did without a permit, if it was SHOWN that incompetent work actually CAUSED the fire, would result in the claim being denied. But every time we ask for some examples, no one has been able to produce a single one. I know of only one case where that happened and it was exceptional, with a lot of documented evidence to support it.
And I don't see anyone here suggesting people do anything illegal, ie install a solar system without a permit and required inspections. As a homeowner, most places in the US, you can obtain the permit, DIY and have whatever work you do inspected.