Those are two bolts/screws that hold the rotor/hub/brake pad on during manufacturing. You can take them out so aftermarket wheels fit properly. Many people have done this and it's perfectly ok. The reason they are only in the front is because you have drum brakes in the back.
Yeah, like Super said, the only reason they are ther is to keep the rotors from falling off of the car while it's 30 ft in the air and hitting someone on the head. Don't worry about it. You shouldn't have any problem with the rear.