Check out this site for more information on this map.
Men were not always allowed to remove their shirt in public- male ‘barechestedness’ laws started to become relaxed after an especially hot summer in the 1930s, and up until the 1960s, men could still be fined for taking off their shirt in public spaces like Central Park.
A call for gender equality led many states to amend their topless nudity laws for women, but even though these states technically allow women to be publicly topless, there are many exceptions to this law. There is a fine and often subjective line between what is legally acceptable and what is considered offensive / lewd behavior. Even when a state is top-free, a municipality within that state may ban female nudity… and in cities like New York where toplessness is allowed, the law enforcement is not always aware of the laws. There have been a few cases of women suing NYC/the NYPD after they were mistakenly arrested for public nudity.