Allegedly Oracle did this years ago when they had layoffs-- employees were let go, then asked if they wanted to return. Apparently, if they said no, the company considered the exit a resignation instead of a layoff, and would refuse severance. If oracle was then taken to court over the lack of severance by former employees, those employees' refusal to rejoin the company could be used to avoid an unfavorable ruling for Oracle.
My coworker relayed his oracle layoff story to me, which is where I got this bit of info. Would love if someone else had a verifiable source.