I have to ask here too. It definitely doesn't imply "US workers from 50 countries". Your change swaps "banned" around. Same difference.
I think that generally the accepted parse is the greedy one where modifiers are as close to their objects as possible. So "Banned weed killer" gets compressed to an object X. That leaves us with "US Workers say X in 50 countries is giving them Parkinson's". How would you interpret that?