Arthur Weasley said that he can make a Ford Anglia fly legally because he did not intend to fly the car, but how is this legal? The car is still a danger if it ends up in Muggle hands. Why is he allowed to do this?
how is this legal? ... Why is he allowed to do this?
Because Arthur Weasley himself wrote the law. AND the loophole. Just to benefit his hobby.
The legal reasoning is as follows:
You have to remember that the whole point of most of Muggle-related laws is one and the same: preventing Muggles from learning about the Wizarding world.
“It’s all to do with bewitching things that are Muggle-made, you know, in case they end up back in a Muggle shop or house. Like, last year, some old witch died and her tea set was sold to an antiques shop. This Muggle woman bought it, took it home, and tried to serve her friends tea in it. It was a nightmare — Dad was working overtime for weeks.” (Chamber of Secrets)
Misuse of Muggle Artefacts is most likely the same, since there appears to be absolutely zero point to prohibiting bewitching "Muggle" objects if no Muggle would ever come across them - after all, many, if not most Wizarding magical objects are indeed just that - a bewitched Muggle object (including but not limited to: Hogwarts train, Broomsticks, self-cooking kitchenware, etc...).
As such, indeed, bewitching Ford Andlia to fly and never flying it (thus not having Muggles discover a flying car and sending Agent J to investigate) would fulfill the point of the law and thus is a possibly OK "loophole" to have.
AND... the real reasoning of course, as noted by Mrs. Weasley, is:
“Yes, Arthur, cars,” said Mrs. Weasley, her eyes flashing. “Imagine a wizard buying a rusty old car and telling his wife all he wanted to do with it was take it apart to see how it worked, while really he was enchanting it to make it fly.” Mr. Weasley blinked.
“Well, dear, I think you’ll find that he would be quite within the law to do that, even if — er — he maybe would have done better to, um, tell his wife the truth… There’s a loophole in the law, you’ll find… As long as he wasn’t intending to fly the car, the fact that the car could fly wouldn’t —”
“Arthur Weasley, you made sure there was a loophole when you wrote that law!” shouted Mrs. Weasley. “Just so you could carry on tinkering with all that Muggle rubbish in your shed! And for your information, Harry arrived this morning in the car you weren’t intending to fly!”
Having said THAT, you can see that this is really just rationalization, and indeed Arthur Weasley's loophole would probably be found "unconstitutional" (if there was indeed a judicial review of laws in Harry Potter universe) - because if Weasleys die, as noted in above quote, there's a risk that Flying Ford Anglia - or other items - would end up being seen by muggles. Or for that matter, if someone (*cough* Ron *cough*) steals it for a joyride.
Fred laughed. “Yeah, Dad’s crazy about everything to do with Muggles; our shed’s full of Muggle stuff. He takes it apart, puts spells on it, and puts it back together again. If he raided our house he’d have to put himself under arrest. It drives Mum mad.”
... and Mrs. Weasley seems to agree:
“You could have died, you could have been seen, you could have lost your father his job — ”