I suspect it's not because of some nefarious plan to "avoid paying their employees properly" but rather it's more tax efficient to pay for their employees' education (because the compensation isn't subject to payroll tax?).
That's correct. Amazon can deduct the education benefit as a business expense and up to $5,250 is not subject to payroll tax. Neither the employee nor Amazon have to pay taxes on anything up to that sum.
Given the continuing-to-soar cost of education, I'd expect Congress to considerably increase that number. People will increasingly get chained to jobs for education cost reasons like they already are for healthcare cost reasons.