I mean they were as safe as they Gould be In the empire and when Pompeji was destroyed there was no credible threat around that justified keeping those walls up and maintained - and apperently they existed - so why is that ? What am I overlooking ?
Pompeii is a Campanian city, not a Latin one.
The city walls at Pompeii are considerably earlier than the destruction of the city. The first city walls at the site are from the 6th Century, which was replaced by a more solid construction at the beginning of the 1st Century, BC. It's not at all unusual for Italian cities to retain their city walls well after they ceased to be useful--it took effort and money to tear down city walls, with little actual benefit. In some cases it might be a serious disruption, as at Pompeii where some people actually built houses on top of the old city walls. Consider as well that Pompeii was destroyed only a hundred years after its revolt during the Social War--those walls had served a practical purpose not that long ago.
During the early days, it wasn't one unified smooth empire (and it arguably never was). Each city was treated differently and had different rights. Their loyalty was far from absolute. Having walls permitted the cities to have more independance. Also, walls require very low maintenance and are useful for non-war stuff (like controlling access to the city, if need be). So the question is: why would they put so much effort in destroying a useful wall that costs almost nothing to maintain?