Why is it so easy to steal art in Europe?

Since 2002, more than $676 million worth of art has been stolen from European museums. Maybe it's just me, but this account of yesterday's art heist in Zurich makes it sound too easy:

Three thieves, wearing dark clothes and ski masks, walked into the Emile Bührle Foundation, a private collection housed a couple of miles outside of Zurich's city center on the shore of Lake Zurich, around 4:30 p.m. on Sunday.... While one held a pistol and ordered visitors and staff members to lie on the floor in the main room of the museum, the two other men removed the four paintings from the wall.... Their total worth is estimated at $163 million.... After the theft, the men fled in a white car, with the trunk open and the paintings visible."

These guys didn't crack a 16-digit pin code or jerry-rig a pully system using shoelaces and chewing gum, mind you. They walked into a museum in central Zurich, in broad daylight, took four paintings off the wall, put them in their trunk, and drove away. I've seen stick-ups at my local DC-area 7-Eleven that were more elaborate. Shouldn't it be a tad more difficult to steal several hundred-million dollars worth of Van Goghs and Monets than it is to jack a Twinkie from the Quickie Mart?