Why is this an issue? Because these Aqui-Hires often end in losses to investors of the start-up, and the start-up's investors may feel that they should share in the value of these stock grants.
It's an interesting issue, and Mr. Arrington's column explains it well. Here's the link: http://finance.fortune.cnn.com/2012/04/25/startup-investors-may-request-acqui-hire-protections/