It's actually worse than that--you can have an option that's an ISO if exercised within 90 days of termination, and otherwise an NSO if exercised outside that

It's actually worse than that--you can have an option that's an ISO if exercised within 90 days of termination, and otherwise an NSO if exercised outside that period (say up to a year, or even 10 years). The only reason to not do it that way is because you hope some leaving employees will just let the options lapse.