The original Six Flags theme park was Six Flags Over Texas. The name referred to the six different countries that have governed Texas - Spain, France, Mexico, the Republic of Texas, the United States, and the Confederate States - and the park retains a theming based around the state's history. The Six Flags company has opened many more parks in other states throughout the United States, but needless to say, none of those states have ever had six flags over them- nor do the parks have any real theming.
The Busch Gardens parks in Williamsburg, VA and Tampa Bay, FL got their names after the Anheuser-Busch brewing company which also owned Sea World. In 2009 AB sold their entertainment corporation to the Blackstone group but the parks are still permitted to use the Busch name.