This is true, I did it in the late 60's. I was probably about 12.
My buddy and I had bows and arrows and enjoyed going hunting for anything that moved, but mainly rabbits. Our travels once took us near a large city zoo. Since we seldom wandered that far from home, we decided we should pay a visit. As I recall, it was a Monday and entry was free. A good thing because of course we had no $$. Since it was a free day, there was no one monitoring the gates and we walked in armed with our bows and arrows. We spent considerable time walking around and checking out the animals. Not once were we questioned, stopped or challenged about anything. Fortunately it never crossed our minds that there were cages full of live targets. We left, walked home and called it a day.
I can't imagine what would happen today.