We're not talking about eating meat; we're talking about crimes against humanity.

And you would have had to break this law in a territory that also was a signatory of the Rome statute. This isn't some out of jurisdiction or ex post facto form of law.

So for instance, US military personnel couldn't be arrested up for crimes that occured in Iraq, because Iraq is not a signatory of the Rome statute.

So your example doesn't make sense unless you ate meat somewhere else that it was also explicitly banned, and then went to another country that had the same cross agreement to arrest people in the combined territories.