At will applies on a one by one basis - at a certain headcount, WARN legislation applies https://www.dol.gov/agencies/eta/layoffs/warn

And the other part is the "goodwill" you mention - if you jettison people for nothin en masse, word gets out, and make it hard to hire in the future

A layoff of any real size has negative impacts to their unemployment insurance premiums and restricts their ability to file PERM for employees. Meeting those targets with voluntary separation prevents those impacts.

If you lay people off, you’re on the hook for their unemployment. A voluntary buyout avoids that, and also avoids mandatory reporting for layoffs.

Voluntary reductions do not have the same reporting requirements or projections that actual layoffs have.

I think both parties probably benefit; people who are more likely to take the offer are those who have another reason to leave (unfulfilled, etc). This will benefit the company in the long run because they can eventually replace them with someone who want to join (people applying for the job).