Referring back to my statement on being a referee myself (volleyball btw). The rules clearly state that the coach should remain within his coach box at all times, sometimes he steps out of it to give an instruction in between points. Rule clearly breached, no disadvantages so I am not going to throw around warnings and/or cards. In the spirit of the game.
Wherever there is a human involved in judging, there is space for lenience. As long as nobody is disadvantaged and nothing got in danger, you can allow a rule breach in the spirit of the game/sport/race. Being a steward/referee means understanding the reasoning behind a rule and interpret if that underlying reasoning is applicable or not.
With this line crossing, the rule is there for safety matters (and perhaps pitstop faking for cars behind) and therefore the stewards have more room for a 'more freely' interpretation of a rule. Was safety endangered? No. Lighter/no penalty required.
Then I take it, I could, in a theoretical situation as a volleyball coach, not care about the the box, as long as there won't be any additional advantage/disadvantage? Though, then again, that's entirely down to the individual steward(referee) ain't it?
That's what you'd ideally like to not happen with clear cut rules.
And in 6 previous instances in F1 where the crossing of the white line after bollard, or crossing of the white line with no bollard this have happened:
In race situation:
Two Drive-Trough penalties (Perez, Monaco and Massa, Interlagos).
Two 5 Second penalties (Räikkönen, Baku and Wehrlein, Barcelona).
In free practice:
One 2500 EUR Fine (Räikkönen, Montreal)
One Reprimand (Hamilton, Silverstone)
When it comes to advantages/disadvantages, that becomes a whole own area of "what ifs". We don't know, if Hamilton had followed the rules if Mercedes would've believed they'd be able to send Bottas away quick enough for Hamilton to pit or not, he might've stacked up, he might've done a drive through the pit.
What we do know is that there is a precedence for these situations. It is a black&white situation, and for some reason. The stewards did not have it in them to follow that, but rather give Hamilton what's usually been a practice-penalty in this case. And if you think strictly safety, Massa at Interlagos was not dangerous IIRC, neither was Wehrlein at Barcelona, neither was Räikkönen in Baku as he pulled out as soon as he saw the line, however that had the potential to be dangerous. Perez at Monaco was not dangerous either, but it was noted he did impede another driver when pitting late.
I also find it interesting how the FIA are more lenient when it comes to incidents behind the safety car, just like Vettel at Baku last year. Making a mockery of the whole point of a safety car. Then again, they do not enforce the double waved yellows either, and we all know how that ended up.