We aren't really sure what causes the bottle caps to become embedded in the pavement, but they get there. We're just glad that they aren't quarters glued to the road or we'd be really sad at all the wasted money. We think it has to do with a combination of heat, people littering, and then cars driving over them repetitively. First, it has to be hot so that the pavement gets a little mushy (it wouldn't really melt or we'd all die since asphalt tends to melt at around 300 degrees Fahrenheit), the heat will also cause more people to buy soft drinks, and because soft drinks sold in bottles are cheaper because they refill the bottles, more people buy those, and because "garbage can" means very little in this place, the bottle caps will just end up on the ground somewhere. Should that unlucky bottle cap end up on the street, it will be mutilated by an onslaught of cars driving well over the speed limit which will in turn slowly push the bottle cap into the asphalt.
Anyway, the bottle caps are impossible to clean up now -- they've fused with the pavement.
This is a more popular phenomenon in front of stores (because they sell things such as bottles of pop). The main road in front of the University of Jordan also sports these beautiful bottle cap accessories.