Who buys all those books politicians write? Politicians become millionaires with those books. Even Kommisar Bernie Sanders exploited capitalist greed and avirace to sell books that bought him an island. Seems like such a thing could be a backdoor money-laundering kickback scheme.
Elected officials should not be allowed to collect fees and royalties from books they write or publish while holding public office. Those monies should go to the public treasury.
In the dreaded private sector, employers own you lock, stock, and barrel. You may have to forfeit gains from outside endeavors to the employers, depending on how the employment agreement is worded.