Another possible reason why Dish doesn't use PSIP data is that it still isn't very reliable in some markets, such as the one I'm in. My DLP has had a few issues with bad PSIP data causing the ATSC tuner to either drop the channel or even lock up. I've also had nothing but problems with the integrated TV Guide On Screen that may be related to PSIP data issues as well.
With their own guide data, Dish only has to account for one source of data, a source they control. If they were to roll PSIP support into their guide system, they would have to integrate data from hundreds, if not thousands of data sources and hope that each one has properly implemented PSIP. If even a small percentage of local broadcasters hasn't, think of the support calls that would generate. Though a bit dated, I found this article to have an interesting take on the situation:
http://www.cedmagazine.com/article/CA240775.html?industryid=43677
Keep in mind, there are a LOT of smaller markets that are just starting the transition to ATSC. These small market TV stations most often lack the money and technical expertise to properly implement PSIP right out of the gate. My hope is that as things improve with PSIP, Dish would at least make the use of PSIP data an option alongside their own guide data. But, until the technology matures, I'm concerned that with PSIP, we'd just be exchanging one problem for another; incomplete guide data vs. unstable and incomplete guide data.