There's not another viable test though that can act on a shorter window than that. Companies aren't out of line for not wanting their employees to be working while under the influence of marijuana - the problem isn't that they're being too strict ('zero tolerance'), it's that they don't have a granular test that would suffice.

That, plus marijuana is still illegal in most states. Whatever your personal conviction may be, businesses aren't in the business to turn the blind eye to illegal substance use. They're on the hook for liability.