"Forced" to "volunteer"

My husband's company put him on "forced vacation" for 4-6 weeks. He was then told that if he wished to "volunteer" his time during that 4-6 weeks, he would be looked upon favorably for re-hire. If he decided that he would not work for free, he was told he would most assuredly be laid off after that time. In addition, a previous layoff provided severance packages to those affected. This current "forced vacation" is apparently not being looked at as a layoff so no severance packages have been disbursed.

1. Is it legal to offer severance packages to one group and not the next?
2. Is it legal to threaten severance if the employee doesn't agree to work for free?

This is obviously unethical but I'm wondering about the legality as well. Thanks for your time!