I wonder if the cause of this is the support costs required for the free accounts? Obviously personal GMail is plug-and-play, but for someone who can't spell DNS, setting up a domain account was never quite drop-dead-simple enough. I suspect the goal here is to create a barrier to entry and prevent the inevitable support load of free users.

There was an excellent post about this a while ago, where a developer reported much better treatment from users after charging a token fee for their app.

As a final thought, maybe Google is catching on to the 'charge what something is worth, not what it costs you' way of thinking. Hosted email solutions for enterprise aren't free, and it seems like Google has realized the real value of their product. Also, ~$4/seat/month is pretty well in line with this kind of SaaS offering.