There is another common reason companies send people home as soon as they resign: security. These company consider all dissatisfied employees as security risks

There is another common reason companies send people home as soon as they resign: security. These company consider all dissatisfied employees as security risks, and resigning clearly means you are dissatisfied, so they have policies saying upon resignation you'll be escorted from the building. (Usually, to keep it legal, on a 2 week vacation.) This is not too uncommon in financial services. I consider this flawed reasoning, but the people I've known who where hit by it considered it a nice bonus vacation...