It's outsourced. Google doesn't run those sites, they're maintained by the company that is providing the merchandise, so they're bound to be using whatever package they're comfortable with.
GatewayCDI, the company that handles Googlestore.com (the U.S. Version) has some pretty notable clients on their list... Novell, Gateway, Seagate, and Edward Jones to name a few. They're certainly not the least expensive route to go, but they do tend to offer higher quality products. Much better than CafePress and other smaller providers.