The Red Flag is, among other things, a traditional symbol of workers’ power, dating back at least to the Merthyr Rising of 1831 when Welsh rioters used calf’s blood to stain their flag red. Dan Sanley, 7 September 2015
The red flag was once banned by law in many U.S. States (and still is on the books in many states) but was ruled unconstitutional by Supreme Court in 1931 in ruling against California's law.
California law prohibits the display of the "red flag or any other flag in a public place" as a sign of violent opposition to organized government. No state military group may carry other than the U.S. or State flags. (other states had similar wording) Then in 1931, the U.S. Supreme Court overturned this law (Ref: Stromberg v. California). Other states with the law banning red flags still on the books are Idaho, West Virginia, and Oklahoma (but with exceptions). Dan Sanley, 7 September 2015
The flag burning decision virtually certainly eliminates the legality and impact of all those laws. Bill Trinkle, 7 September 2015
There are still a variety of "banned flags" on books of many states. For example, it is not unusual to see flags of foreign countries banned on school yards or that only flag of USA can be flown. Dan Sanley, 7 September 2015
One needs to examine the hysteria of the times. In the 1920s and 1930s, most of conservative United States had become isolationist, suspicious, fearful, and distrustful after their idealistic entry into World War I and disappointments that followed. The world had become a very fearful place, and they were deeply affected by world events, and the response was the passing of many now-unconstitutional laws that were put on the books at the state level, including those that infringed on the civil rights of individuals. It was the age of the Red Scare, and it resulted in increased discrimination, bigotry, intolerance, prejudice, racism, and injustice in the United States. The rule of the day was to protect our country and traditions against anything that endangered it, including displaying anything considered un-American like the red flag.
Since that time, the Supreme Court has ruled most of these Jim Crow-like laws unconstitutional and void, but many are still on the books, having never been officially removed. It's more like bad bookkeeping, and although they may still exist, they are no longer enforced. Both the United States and state governments, like those in other countries, react to public panic and fears, and sometimes pass bad laws as a result. Not too hard to understand for us today, considering the climate after 9-11 in the United States, some of the resulting laws, and with the continuing senseless shootings and terrorist attacks taking place worldwide. Pete Loeser, 11 February 2016