Sock
is that not cannabis though? Nowhere in the law does it say marijuana unless i am mistaken, and the two are completely different. I wonder how they could Police this as it grows freely. My old neighbours had a plant and I swear they didn't know what it was. I could hear the dw say to the dw won't it be nice when it flowers. It just grew there, a great big humongous weed plant 
Anyway, I think that people should lighten up a bit, its summer.
Maybe the person/s smoking it were young and being a bit daring, it really won't hurt anybody else.
I have smoked weed at family events and seen dozens of parents doing this, ok perhaps not when as tightly packed as the OP suggested, but its all part of the fun of a summer event.
And yes it may be unnecessary to those who have not experienced it but so is drinking to some people. I just don't see the attraction there, it just makes me want to throw up after a couple.
Anyway, my final philosophy on the matter, God said something about the grass being free for us all, so I choose to roll mine and smoke it. He also allowed it to grow, so if its good enough for him. It also has many healing powers and is great pain relief. Lovely in stews, cakes etc.