Copyright laws
Because the President is technically a public servant, everything unclassified that they say write or do is considered to be part of the public domain under copyright laws. So while they are in office, they are not able to officially get a copyright if they want to release a book and so on.
We guess they will have to wait until their time in the White House is up before they launch that new book deal and make a huge amount of money. Usually, most leaders do that, book deals and public lecture tours make them a good amount of cash. We would do it too, why not, more money couldn’t hurt.