For me, there are three items in A, B, C or D, but four items in A, B, C and D, with or without an extra comma. Are the Americans confused between the meanings of "and" and "or" ?
I think there are several valid interpretations of "A, B, C or D":
{A} OR {B} OR {C} OR {D} : "Would you like red wine, white wine, orange juice or water?" (not an invitation to be greedy)
{A,B,C} OR {D} : "Rock, paper, scissors, or toss a coin?" (don't think I'd want to play a game of "scissors or toss a coin")
{A,B, {C OR D}} : "Lasagna, seasonal vegetables, chips or boiled potatoes" (A favourite of all pubs before they went gastro)
I would tend to assume the first unless context suggests otherwise.
With "and", there is little ambiguity as conjunction is associative: {{A AND B} AND C} is equivalent to {A AND {B AND C}}