The D.C. Sweatshirt

Can someone explain the phenomenon of the D.C. sweatshirt to me? I don't see any for 9 months of the year, and suddenly, in April, when it warms up a bit, but isn't warm enough for short sleeves, the tourists descend and out come the sweatshirts. My office is a few blocks away from Penn Quarter, which appears to be the epicenter of D.C. apparel. Tourists hit the mall, buy their sweatshirts and hats (the most grievous offenders are the pink D.C. sweatshirt and the F.B.I. t-shirt), wander up to the Spy Museum, and then leave again. Is it too difficult for people to bring theirs from home? Or has the D.C. sweatshirt become a fashion statement for tourists, like those I <3 NY t-shirts that were so ubiquitous a few years back?