[nradov]

Employee discounts can't be very high or else employees will set up side businesses buying up cheap products and then reselling them on eBay or whatever (arbitrage). This directly hurts the employer's profits. With an employee discount of 10% or so it's usually not worth the hassle. But when discounts go over ~20% it gives employees an incentive to act unethically. Employers can partially counteract that by limiting purchase quantities but it becomes a huge hassle to enforce.