Corruption.
Perhaps him being in custody will lead to some of the money being found and returned.
There's no doubt Maduro enjoyed some taste of the froth, but this is a silly oversimplification.
They also spent billions to influence the elections of the other countries of the region. Chavez was also giving away oil for free in exchange of political support abroad. He even gave free oil for the buses of London because the mayor at the time was in good relations or something.