They can only be popped out from the front. In this way, should you get hit with a racquetball, for example, the lenses will not shatter nor will they eject out the back of the frame. If they did either one or both of these, you would receive a bad eye injury which could cause loss of vision. The lenses are usually of poor quality and frequently cause distortion in your vision. In addition, if the lens is not of good quality, it could shatter and cause you an eye injury during sports.
Ultraviolet protection is essential for sunglasses during serious sports competition.