Basically the concept in F1 teams, or in racing in general is, if it isn’t checked it’s allowed. It becomes cheating IMO when it becomes “I know well it isn’t allowed and they verify you don’t do it, but they verify it so rarely that I consider the performance gain worth the risk”.
My take on a few cases.
The Michelin front tyres :
there is a rule defining maximum width of tread on front tyre at any point during the Event. It wasn’t specified it had to be new but FIA just used to measure it while new because it was more simple to clearly define the tread width.
Then it was discovered that some Michelin teams, it’s unclear if it was because of tyre design or of car setup, were de facto using more contact area than allowed. I don’t believe FIA ever measured the used tyre before so it was a grey area, FIA clarified the matter, but didn’t DQ the teams.
I agree with the decision because for me it isn’t clear cheating, it’s going on the limit, it passes the test you know FIA does, it wouldn’t pass a different test FIA could do if wanted, but never did, possibly simply because FIA never thought about. It’s not in the spirit in the rules but there’s a grey area in the letter. Close the loophole and move over.
Flexing wings :
FIA specifies, with accuracy, which kind of tests the wing will be subjected to. If a wing passes that test it’s legal. It’s obvious that everybody design the wings to pass the test, certainly not to be as rigid as possible. If there are complains, FIA can decide to make different tests, for example increase the testing load, but when that happens teams receive full details of the new tests and will modify the wings accordingly.
That is even less on the limit than the other one. It passes all the tests you are required to pass and you know they wouldn’t do a different test without first telling it. We can argue it’s not in the spirit of the rules, but it’s fully in the letter and that’s what matters.
An important thing to remember.
F1 is a relatively closed world with a limited number of participants. There are several discussions and several clarifications per year between FIA and these participants, most of these discussions we don’t even know about simply because we don’t need to know. All these clarifications are then de facto part of the rules because they define how you should read a given rule. The rules you download from FIA.com alone aren’t enough to know if a given thing is legal or not, because these don’t include all the clarifications hence don’t give you the whole picture, just an idea. They are good for example for bodywork basic dimensions, but you wont find in them what is possible or not possible to do in term of launch control, although you can bet that this is an area where several clarifications exist.
Example the BAR case last year. If you take a look at the rules from FIA website you can argue for days about the fact that it isn’t clearly specified for car to weight minimum 600 kg without fuel.
Still that’s the way it works because, several years ago the issue was clarified. When BAR case exploded I heard from an engineer working at Minardi in 1994 that that year, looking at refuelling times and car performances on the track, they noticed that something was wrong with Jordan cars, it looked like they were adding more fuel than needed at the refuelling, presumbably using part of it as ballast to go over the minimum weight at the end of the race. They complained and FIA clarified back then that that wasn’t possible and since then the car must be over the minimum weight, now 600 kg, when you remove all the fuel. Since 1994, although not frequently because it takes long, FIA does weigh cars without fuel. It doesn’t matter then if BAR lawyers wrote a long report in an attempt to demonstrate that they were always over the minimum during the race hence it wasn’t cheating but a different interpretation of rules.
The fact is that the car, without fuel, was under 600 kg, and FIA clarified long ago, and AFAIK confirmed several time, that that’s not how the rule is intended.
That’s typical case of cheating.
Another matter is that apparently there were at least 3 further teams doing the same but here we start to talk about politics and I prefer not to go in it. (anyway if you want some hints... AFAIK it was silver, yellow and Red... notice the capital R

)