The reason for this is because the HR person drafting this policy probably had someone take advantage of the window between when someone is told they are laid off and when they are actually terminated, and so the policy gets made to reduce that liability. It sucks for the 90% of people who wouldn't do anything, but that's how it works. Imagine getting told you're your laid off, then your laptop handed in before you even leave the meeting room, then a security guard or HR person escorts you to your desk to pack your things in a box before they walk you out the building.
Mind you, you'd probably still get some severance bonus or keep getting paid for weeks or months, but you just wouldn't be expected (or able) to work.
Because of labor laws in the UK, HR did everything they could to avoid laying off the UK office staff because the process and cost was prohibitive. We kept people in the UK office for years just because it would have been a pain in the butt to remove them even if they were redundant or basically had no tasking.
I understand that most(?) Of the US is an "at will" jurisdiction, but where I'm from all employment contracts have a notice period in them. So when employment is terminated, by either party, there is a 2-4 week period where the employee works out their notice.
Often in the case of lay offs there would be plenty of heads up before the official notice period as well.
Only in the most egregious cases of misconduct could an employer get away without a notice period.