Worries me even more since we now travel with a service dog. I see it often, just on the weekend in fact. The campground owner's comment was, "Oh, that's my neighbor's dog. He comes over here all the time."
It's distracting at best, but can be much worse. May sound overly-dramatic, but I know of incidents where not-so-friendly dogs attacked service dogs, no matter how well-behaved their clueless owners trusted them to be while off leash. In one case, a $25,000+ animal was seriously injured while on the job, leaving a disabled person without assistance.
Leashing is so simple. But we all know that the world is full of people who think that rules don't apply to them.