Why would you think they would not be able to keep the money? You wrote 'former' so they aren't employed as a shill while working for the gov't..they're civilians again, so where's there an issue?
For example, Clinton, after he left the Presidency, was paid (mighty well) on the lecture citrcuit for speech-making. Others have done the same. To my knowledge, any commercial advertising they do is legal and they are entitled to get paid.