Not all paints are flammable these days, of course. But the rules are there - they almost certainly date from the days when most paints were flammable. Same as pacemakers in bulk are still considered hazardous cargo, because for a while in the late 1970's some were made with radio active batteries to give a long life. They didn't prove very popular ( surgeons prefer pacemakers that have to be replced every few years when the battery gets low, because that way, they get more money) and pacemakers haven't been made with those batteries for at least 25 years. But IATA still has them as hazardous....