I don't think a dishwasher can have a check valve as they are designed to remove food particles. A check valve might get clogged pretty quick under those use conditions.

I'm talking out of my ass ofcourse, some models might have those valves since some models have a built in high loop. But code is code, and it's meant to protect the average user. I think the manufacturer would have to send a lobbyist to ask every state, city, and township to update their code as well.