If the employer can't find a way to offer any incentive other than money to earn an employee's loyalty, then they're going to lose talent that way. The vesting cliff isn't a probationary period, it's deferred compensation. Leaving after vesting isn't any less ethical than having a vesting cliff in the first place.

A better question is, if good workers leave after they hit the vesting cliff, what's wrong with the company?

Based on my experience, I'd say that this is endemic to the ass-in-seat... I mean, IT industry.