Dunno, I got the impression that the rules were not actually a thing here. IIRC neither diane or king were that much aware of the rule ban spoke of. And in principle, the concept of the very rule ban mentioned is stupid and impossible. The sins are known as dangerous criminals who committed heinous crimes. Their sins are not private matters which concerned them, their sins are large scale public events which affected hundreds of lives each. In this particular context ban suggesting their sins are private matters is simply downright moronic. I mean, king was sentenced as the king of the fairies who allowed his subordinate to kill god knows how many for 500 years. Meliodas is blamed for turning an entire nation into a freaking crater. Ban is blamed for murdering a saint and destroying a kingdom. What ban suggested gets even more downright stupid if you consider that so far none of the sins actually did that. In fact, they are innocent and victims of the whole things (at least ban and king are, we don't know the details of what happened with meliodas). I mean, ban befriended the saint and saved the forrest (the exact opposite of what he was blamed for) and king did not have his memories for 500 years or so so he really didn't allow anything to happen, there was never anything he could do about it. So ban is suggesting that they keep to themselves, let everyone think they are all horrible criminals instead of telling people what actually happened which would actually show people that rather than horrible criminals they are even greater heroes than everyone already thinks they are. I can buy the wins wanting to keep to themselves but sticking to the rule is stupid when the alternative is just let everyone think you are responsible for thousands upon thousands of deaths at least.
Seriously....
Some guy: Whoa, you are meliodas! You are the guy that murdered everything in that kingdom for the lolz!
Meliodas: Actually.....
Ban: Shut up some guy! You are not allowed to inquire into the matters of the sins!
Some guy: Inquire? This guy is a cold blooded murderer who does not give a crap about anything, I don't want to inquire anything. I want to run but my legs are paralyzed with fear and my brain tells me that running is presumably pointless anyways. I mean, have you seen danafor? This guy turned it into a crater. You can't even see the button of it. I can't outrun a massively evil force of darkness with the power to punchbliterate nations.
Meliodas:
Actually my sin isn't the destruction of danafor but...
Ban: CAPTAIN! THE FUCKING RULES!
Meliodas: Oh, all right, I will let him think I murdered everything for literally no good reason.
Some guy: Holy crap, you are ban! You killed the saint of the forrest and burned it down with all the fairies in it? And they say you also raped the body of the saint. And then you sold the bodies of fairies in the black market. And you chop of your private parts and throw them at old ladies walking down the street. Somehow repeatedly.
Ban: Actually...
Meliodas: BAN! THE RULES!
Ban: The rules are stupid.
**throws penis at the guy**