Yep, you read that correctly - it is illegal to yarn bomb. It is no different than graffiti or any other type of street art because it is still being placed on public property without consent... not to mention the dangers that the yarn may place on the wildlife. Yes, I said dangers. No one ever stops to think that the animals will eat or chew on the yarn. Knitted yarn can unravel in the stomach and intestines causing the intestine to rip if the yarn gets stretched at all, killing the poor animal.
Think before you decide to "beautify" an area by yarn bombing... why not just make a nice pair of socks instead. You can 'yarn bomb' your feet - keeping animals safe and it's totally legal to boot!
I do believe that all art is beautiful - even the ones that I don't understand- but I also believe that criminal acts are illegal - even the beautiful ones.