I've worked in MA, RI, CT, NY, NJ,PA. I understand the issue with NY, but what about the other states - shouldn't Box 16 only include earnings in the respective states?

You should have a separate entry for each state, showing how much was earned in each state and how much taxes was withheld for the respective states.

If it is not done on the W-2, it will have to be done for the respective state tax returns.