Forbes' Top-Earning Deceased

For the first time in at least five years, Elvis Presley did not top Forbes Magazine's annual "Top-Earning Dead Celebrities" list. Kurt Cobain took the honors this year (October 2005-October 2006) earning $50 million. To round out the top 13 list:

2. Elvis Presley ($42 million)
3. Charles M. Schulz ($35 million)
4. John Lennon ($24 million)
5. Albert Einstein ($20 million)
6. Andy Warhol ($19 million)
7. Theodor Geisel (Dr. Seuss) ($10 million)
8. Ray Charles ($10 million)
9. Marilyn Monroe ($8 million)
10. Johnny Cash ($8 million)
11. J.R.R. Tolkien ($7 million)
12. George Harrison ($7 million)
13. Bob Marley ($7 million)

I wonder what role intellectual property laws (i.e. things in the public domain) plays in how much these deceased figures make. After all, this list does not include anyone who played a key role in the 18th or 19th centuries and before (e.g. James Madison, Benjamin Franklin, Abraham Lincoln, Charles Darwin, etc.)