A frequent argument of proponents of a strong central government with authority overriding locales is that if states had varying laws, the effect to travelers would be confusing and would lead to more people being jailed for doing things that they didn't know were illegal. Ignoring the completely asinine end of it, that not knowing otherwise, people might assume violent actions were legal, we are left with what to do about varying levels of legislation against non-violent acts. Generally these fall into two categories – economic regulations and moral regulations. I think the following strategy can address this problem:
If it can be shown that a nonviolent act was not illegal in
the subject's locale of residence, and if it can be shown that he had not been previously accused of the crime in the locale where it was committed, then the conviction can be thrown out on the basis of tourist ignorance. Nothing is said about acts which are committed in tandem with the excused act and that do not fall under this measure otherwise.
Yes, this does mean that tourists get one, and only one, ticket out of a nonviolent crime that they may not have had prior knowledge of. This would not excuse any violent crimes committed at the same time, or other nonviolent crimes of which the perpetrator was provably aware of their illegality.