Most rules state to NOT leave dogs unattended.
Tied out ONLY when you are right there.
X2 regarding keeping control of your own dog. I have a Chihuahua. When we are walking her and we encounter someone else walking their dog, we ALWAYS pick her up. It does not matter if owner says dog is friendly!
Yesterday we went for a walk at a very populated state park and a woman told us her dog and someone elses dogs were both attacked the previous day by the same dog being walked by a, presumably, irresposible owner.
The police were called and the dog was forever banned from the park.