To combat obesity should fast food restaurants have scales at the counter which read your weight, and then they refuse to serve you if you're body

That makes no business sense for them.
They're in there to make money. And the more you buy, the more money they make.
They do not care what you weigh.

Self control .. is all that's needed.

Wouldn't worry me, but plenty of people in Australia/US and many other countries would have no or little food

No, to counter obesity fast food should actually be fast. You should have to catch that shit.

Yes. ...one should have to pay an higher tax like fo cigarettes

No. They should give fat burner capsules complementary.

It'd be better not to put appetite stimulants called MSG in the food to begin with.