The 3DS demo units I got a chance to play at comic-con had an attachment on the back of the 3DS preventing you from accessing the cart.
Same with all the demo kiosks, that way people can't easily just steal the cartridge and run off with it. Especially now that anyone with a Gateway can dump their carts.
Technically still possible, but I doubt anybody walked into their Best Buy carrying industrial metal cutting pliers (Strongly suspect that's how all the demo carts get sold on eBay though)