In all the examples you cited, the person may not check "the rules" every time, but they'd better operate in accordance with them. An accountant doing something contrary to GAAP is going to be up a creek when an audit happens. A cop arresting someone for something that isn't actually illegal is likely to get sued, and an FDA inspector that shows up in a dining car filled with people eating food that they brought onboard is going to be less than amused.
Can and should the rules get bent from time to time? Certainly. Just this weekend in Chicago I was carrying a box of delicious leftover Giordano's when I walked into a bar with some friends. The person at the counter correctly said "Hey, you can't have that in our restaurant, you'll have to leave it with me." I said "Of course", and we headed to the bar for a drink.
30 minutes later when it was closing time, the guy came back over to me and said "Hey, it's quitting time for us, I can't just leave this at the counter up front so I'm going to give this to you and you have to promise me you won't sit here and eat it while you finish your drinks". Again, no problem. The guy certainly didn't have to do that for me, he could have either made me leave or toss the pizza, but bending the rules was appreciated.
Like dlagrua posted, I'm sure the rules get bent from time to time in the diner too, and it isn't the end of the world.
But it's completely out of line for a CSR to tell someone something that's the exact opposite of the correct answer when they call it.
(and yes, buddy 559 raises a good point that had crossed my mind - the CSR may have correctly answered the question that they were asked, or there was a misunderstanding between the two of them)