Well actually the CARB cans are just having the opposite effect. The idea was to limit vapor emissions but the contraptions to prevent it cause plenty of fuel to be spilled. Maybe some TV station should go and get the head of CARB, give him/her a CARB compliant can and ask him/her to fill a lawn mower, or for bonus points fill a car with it.
Titan avoids the EPA/CARB quagmire by limiting their offerings to Diesel only. Some other companies put the usual "off-road only" disclaimer on their products but technically you could fail visual in CA with a different tank.
Somebody who has seen a few different gas and diesel tanks might be able to say what the differences are. They do have different part numbers for the same year. Looking at pics it appears the fuel pump is mounted differently.