If I were an employer I'd make sure I never gave a reason. The burden of proof of discrimination would then be left to the employee. Since it's almost impossible to prove discrimination unless it's very overt, this is going to make it much harder for employees to make a claim, not easier.
But conversely it makes it much easier for employers to treat employees as disposable commodities. Which is of course the real point of the plan. Employees will be much less likely to rock the boat or stand up for themselves while they're on probation.
Agnes, I think you're understating the amount of damage this will cause. It's a monstrous broadside from the New Victorians and the only possible effect will be to drive down job security and wages, while also filling companies with time-serving politically adept careerists.