"If you touch me, I'll yell 'rat'!" makes no sense

This line in the movie makes everything Ferris does to get to eat in the restaurant completely futile.

Think about it.

If the maître d' is SO worried about him yelling 'rat', why do they even need to do ANYTHING else but just threaten to yell 'rat' to get anything and everything they want?

"If you don't give us a table, I'll yell 'rat'". "If you don't serve us this food for free, I'll yell 'rat'".

Where does the line go, why would he be so worried about that, but yet not do their bidding if it goes too far? Of course 'free food' would probably be too much, but it's never explained where he draws the line, and how far he could be pushed with the whole 'rat-yelling threat'.

It makes no sense that Ferris doesn't even TRY to push it as far as he can (he even says 'you can never go too far', which is the most psychotic line I have ever seen a movie character say, because even the most villainous characters have SOME kind of morals and line they won't cross - basically this means Ferris would MURDER someone for a prank, when you think about it).

Ferris is the type to push things as far as he can, and then some. So why doesn't he see how far he can go with that 'rat' line?

It makes no sense that he'd make such a threat for only ONE small thing, but then try to go through an unnecessarily elaborate sting operation for everything else.

It still bugs me so much that Ferris says 'this is my 9th sickday this semester' (or something like that), and yet Rooney says Ferris has been sick for NINE DAYS already (which is also what the computer shows).

So which is it? If TODAY is the 9th day, then he can't have ALREADY been sick for NINE whole days. Why do people make this mistake so much? They will say it's your 20th birthday, when you turn 20, although that's CLEARLY your 21th birthday, because your first birthday, you were 0 years old (being born is your first birthday, the most LITERAL birthday of them all).

Why do people get it right when it comes to centuries, but not days? 1900s was the 20th century. No one gets this wrong. 2000s is the 21st century, there was even a studio with that name, "The 21st Century Fox" (though what kind of a fox is that, is not explained).

Yet, when it comes to 20th birthday or 9th sickday, even supposedly intelligent moviemakers get it _WRONG_.

Why is that? It makes no sense. It would make some kind of sense if everyone also got the century thing wrong, but THAT they get right every time.