Why Radiation Doesn't Come Out the Back of the Utility Box
Your electrical utility box is made of and completely surrounded by grounded metal, including the back of the box. The only part of your electrical box that is not grounded metal is the glass part of the smart meter. The glass part of your smart meter is where the RF waves are transmitted from and they're not directional. The RF waves radiates everywhere but can not penetrate grounded metal. Once the Smart Meter Guard is installed it covers that last part of your utility box that is not grounded metal. Once the Smart Meter Guard is in place the high frequency radiation can not escape your utility box and all the radiation is contained.
Here is a simple analogy. Say you have a sound speaker and you put it in a sound proof box that has a hole in it. When you turn on the sound you heard the sound, even from the back of the box. Now, if you cover the hole in the sound proof box you the sound can not escape and you won't hear the sounds. The Smart Meter Guard basically "covers" the hole in your electrical utility box with a "Faraday" cage.
Radiation is very similar to sound except it's dealing with high frequency radio waves and grounded metal.