It is, by now, in the "public domain". That means it is public property and can be used by anyone for any reason. Copyright laws in the US have changed over time, but since Treasure Island was published prior to 1923, it is no longer protected by copyright laws and is in the public domain.
Copyright laws protect a particular work from being used or exploited by others. They protect a particular piece of work, but they do not protect the idea behind that piece. In other words, the idea behind Treasure Island could be used by other writers as long as they expressed the idea in a way different from Stevenson's.
Copyright laws protect a particular work from being used or exploited by others. They protect a particular piece of work, but they do not protect the idea behind that piece. In other words, the idea behind Treasure Island could be used by other writers as long as they expressed the idea in a way different from Stevenson's.