Why is gun legalised in US?

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Columbine high school massacre,Platte Canyon High School shooting,Dawson College shooting... Just to name a tragic few incident. The root of all these problem? Those maniacs who open fire or the companies that sold the the firearms to the hands of the maniacs? Isn't it ironic that the government comes out and condemn those action but yet still legalise the sale of firearms? puzzling...