I'm not sure about that, maybe it is based on the definition of "safe". There are tortilla chips made in Chicago that explicitly say they cannot be sold in California on the packaging. This is due to chemicals banned in Prop 65.
https://www.reddit.com/r/mildlyinteresting/comments/1220zn9/...
CA can't say "the factory that made these chips emitted chemical X into the air during manufacturing, but none of it is in the final product", so you can't import it.
The Federal Government can, but not an individual state.