Because, in fact, the sale of gas is subject to a great deal of regulation. That's to prevent someone from selling you adulterated gas that destroys your car's emission system. Or from selling you leaded gas that pollutes the air that our kids have to breathe.
And the reason the manufacturer doesn't need to enforce your use of the roads is that it's already being enforced by a higher authority. We have cops for that. And they very much do dictate that you keep your car on public roads, and not go driving off across someone else's lawn, or the National Mall.
If you think these extensions to the metaphor make no sense, you're missing the elephant in the room: Personal computers are insecure, and the average web surfer is more likely (probably, alas, by an order of magnitude) to have their computer steal their credit card numbers or grind to a halt under a flood of malware than they are to crack open the box or write a single line of code. An enormous number of people don't want the freedom I want, any more than they want to own an acetylene torch.
Well, like all metaphors, it's only an approximation. However, the laws don't dictate that I keep my car on public roads, they dictate what I do with my car when I'm not on my property. And while those laws are enforced by a higher authority, if you follow the chain up in a democracy, you come to me again, in theory.
To your other point, Windows is insecure, not personal computers. A true personal computer is/would be owned by me, not a corporation, not a hacker.
Hmmm... Seems to me I've seen this freedom vs. security argument elsewhere...