First, they can cite you for anything they want. Whether or not the citation is valid is for the court to determine. Some municipalities collect a large part of their overall revenue from citations issued to people “passing through” from another state, particularly since many of the out-of-state drivers don’t want to go back to present a defense.
It is unreasonable to expect an officer to be familiar with the regulations of every other state. In fact, I’ve encountered officers who weren’t familiar with the more esoteric regulations in their own state!
For the case of private passenger vehicles / motorcycles, if the vehicle is legal to operate in its state of registration, and the owner still maintains residence and garages it there, then reciprocity makes it legal in the other states.
This pertains solely to equipment. Operation is subject to the laws of the state the vehicle is operating in. Some examples:
[ul][li]Car: No front license plate - permissible if the state of registration does not require one.[/li][li]Car: Headlights off when raining - violation, assuming the state the vehicle is operated in has a “wipers on - headlights on” law.[/li][li]Motorcycle: “Ape hanger” bars - permissible if allowed by the state of registration.[/li][li]Motorcycle: No helmet - violation if the state requires helmets be used.[/ul][/li] Delaware is weird - you have to have a helmet with you, but it doesn’t have to be on your head.
The rules for commercial vehicles and other types of vehicles (police cars, for example) are different and are too complicated to get into here.