The original intent of the free DTAs was to compensate for moving most analog channels to digital, making direct-connected cable-ready TVs useless. It seems it also gave Comcast a chance to sneak in yet another price increase once customers got used to using them.
What you say is true for @ezgoin, who has "Digital Adapter Additional Outlet Service 1st & 2nd Outlet (subscribed to before July 2012)" . The 2013 fee structure in the January bill shows the change in price from $0.00 to $1.99. However customers who have "Digital Adapter (Limited Basic only)" are still no charge.
Comcast told Limited Basic customers that the DTA was "free", and (true or not) that it was "required". Limited Basic customers should complain to their cable franchising authority if ever the DTA is not free, because basic services have some special rules still. Not that it matters, because Comcast simply raised the price of Limited Basic by $2 instead! (And now another $1, to $23/month as of January 2013.)
But clearly Comcast are angling to get further revenue even out of basic customers, since they have touted "Beginning 01/07/13 you will receive most of the channels on your digital adapter that you receive on your set-top box, with the exception of On Demand, premium, and HD channels. There is nothing you need to do, except enjoy all the additional programming." That additional non-broadcast junk is going to be their wedge to attempt to raise prices more, rationalizing additional content to the franchise authorities (towns). Combining this with the newly-permitted encryption of broadcast channel redistribution, plus the loophole allowing non-HD broadcast redistribution, is now making even direct-connected digital-ready TVs useless! This is a good reason for every Limited Basic customer who was originally a Broadcast Basic customer to try again with an over-the-air antenna at their location.
The only price that decreased in January 2013 was "Additional IP Address (each monthly, up to 3 additional)" which is surprising, because we're running out of IPV4 addresses globally!