What is the rationale of promising to not reapply? If a company is hiring again 6 months after some layoffs, what would be wrong will rehiring an ex-employee who was one of those laid off, assuming they are the best candidate for the new job opening?

Some places would also love to get the people they lay off back, at some point in the future. I believe Salesforce did this. Obviously the company matters and each one will have their own stipulations.