Scientific is the adjective form of science, and American is a noun meaning a citizen of the United States. The intended audience of the original Scientific American, first published in the U.S. in 1845, were people with either a professional or avocational interest in the many fields of science. If you fancied yourself a person of science, you were the target audience. The magazine today still boasts, "A third of Scientific American readers hold postgraduate degrees."
Also, the magazine was actively involved in the early patent process in the U.S., helping scientists and inventors protect their inventions and disseminate their ideas. According to the magazine's website:
In an era of rapid innovation, Scientific American founded the first branch of the U.S. Patent Agency, in 1850, to provide technical help and legal advice to inventors. A Washington, D.C., branch was added in 1859. By 1900 more than 100,000 inventions had been patented thanks to Scientific American... For a century, Munn & Company retained ownership of the magazine, which chronicled the major discoveries and inventions of the Industrial Revolution, including the Bessemer steel converter, the telephone and the incandescent lightbulb. Edison presented the prototype of the phonograph for inspection by the editors, and Samuel Morse, father of the telegraph, and Elias Howe, inventor of the sewing machine, were frequent visitors to the offices in downtown New York City.