Cherry picking. Each flag is its own thing, with its own baggage. They are, most of them, devices to convince people do horrible things without feeling personally responsible for doing them.
The French flag now is the same as the flag of the First Republic, and the one that flew over the Reign of Terror. The Turkish flag, as the flag of the Ottoman Empire, flew over the Armenian, Assyrian, and Greek genocides. The Soviet flag flew over the genocides of Cossacks, Ukrainians, and Chechens, and the mass deportations of Lithuanians, Latvians, and Estonians. The 24-star US flag "Old Glory" flew over the Trail of Tears.
If you banned a flag every time something horrible happened under it, all of our flagpoles would be empty, not just the people from the American South. But then, the next time a group wants to come together and do something horrible without bearing a burden of guilt afterward, they will just hoist an all-new flag to shield them from their shame.
Or maybe they will do something great, and share that glory through their shared standard. It has happened, at times. Digging the Suez canal. Digging the Panama canal. Landing on the Moon. Establishing institutions to support the common welfare. Building transcontinental railroads. Universal literacy campaigns. Some of those things were made possible by a manufactured sense of ideological kinship.
Let us not forget that flags are just symbols, and identifying marks, whose meanings are interpreted entirely in the eye of the beholder. The flag that is placed on Memorial Day, on the grave of a man who died under it, may look the same as one that is carried at a white supremacist rally, but only one ought to be offensive to a descendant of Confederate-owned slaves. We do not have a duty to shield others from emotional distress, by attempting to prevent the latter from being sold.
If you ban one flag, you either have to draw a line around acceptable levels of offensiveness that originated under any given flag, or you have to ban them all. Deciding where that line goes is very difficult to do fairly.
Funny you don't make more apt comparisons, like a Waffen SS battle standard, or an ISIL flag.