This was called Seward's Folly because there seemed to be no good reason to buy Alaska.
William Seward was the Secretary of State of the United States at the time of the purchase (1867). This means that he was the one who negotiated the treaty. It is for this reason that it was called "Seward's." The treaty was called "Folly" because some people believed it to be a very bad deal for the US. They felt that Seward had paid Russia millions of dollars for a huge chunk of frozen and worthless land. For that reason, they felt it was foolish and called it a "folly."