These policies are BS, what they really are is the unlimited right to negotiate for vacation time. They are also frequently chosen because they mean employers don't need to cut employees a check for vacation time not taken when they leave. When I left my last job which had this policy I asked for a check for vacation not taken, and was told that one reason they chose that policy was that they wouldn't have to cut such checks.
Any reasonable employer can make exceptions to a fixed vacation policy for things like sick leave, if an employee gets layed up for a month after a car wreck they can choose to pay them despite the policy.
Either type of policy can be interpreted generously or not, but with fixed vacation policies there are at least limits on how poorly you can be treated, and you know it up front.