No they are not. If you and I are standing next to a firearm being shot and I am wearing earplugs while you are not, do you think my earplugs are going to reduce the sound of the shot for you?
I'm not sure how many times this needs to be said. Earplugs block EXISTING noise from reaching the wearers ears. Earplugs have ZERO effect on the noise produced by the firearm itself therefore thy cannot be deemed as Prohibted device.
Maybe you have terrible ear plugs, but mine effect the firearms noise.
Lets try this example then. I crank up music to 120-140 dB and then shoot, the music is now a prohibited device because it muffled the shot! And now the sound waves are touching the firearm