you dont just ‘forget’ to renew your law license, or ‘let it expire’ that would be incredibly stupid after spending years ain law school and then passing the bar exam...
They voluntarily surrendered them, usually an event that take place about 5 seconds before tyhey are about to take it from you.
I have asked several lawyer friends about this... Who told you that? If they did, i am sure they would be the first
Many people who have law degrees but who do not intend to continue practicing law deactivate their licenses rather than risk losing it for being in arrears.
Michelle Obama opted for inactive status and Barack Obama opted for retired status.
She could reactivate whenever she wants, but he would have to reapply for licensing. Which implies that she thought she might work as a lawyer again one day, while his action implies that he felt he was done with lawyering.
Neither "voluntarily surrendered" their licenses.