Also I imagine that their current way of doing things generates more money. As it currently stands, if you'd rather see the uncut version, you can see the PG-13 version in theaters and then have incentive to buy the DVD later, what with its extra content and all. Under your method, people can just see the version they want the first time and have less reason to buy the DVD. 

Not that I'm an expert on this. Just a thought.