Normally, the people who retire with clearaces like these are honorable so offering them that courtesy isn't an issue.
I could be wrong but I don't think they actually have access to the systems where the classified information is kept. Everywhere I've worked in the private sector has disabled my email accounts, computer passwords and asked for all proprietary business materials when I've left. They didn't have to because I'm not a theif or spy but that's the way it is done.
When there is a higher up who the company wants to keep in the loop after retirement, they usually create a paid consulting position if they want to maintain access.