Look at it from his perspective. He knows what the IPO is going to value the company at now. He's looking at some low-performing employees and calculating that

Look at it from his perspective. He knows what the IPO is going to value the company at now. He's looking at some low-performing employees and calculating that they're simply not worth keeping around if it means giving them $20 million (or whatever) in the IPO. If it's ethical to fire these people (in which case they lose all unvested shares) then it stands to reason it would be ethical to renegotiate a deal that both parties think is fair.