Soooooo many questions... Will they have an FFL on hand to receive, log, and dispose of the guns? Does the charity have to pass a background check and wait 3 days? How many other ways could this possibly be illegal?
From their web page:
"Volunteers destroy firearms in your presence"
Basically you hand then the firearm, they place it in an industrial crusher while under your direct observation/supervision. There is no transfer of a firearm taking place.