Frustration?

This morning we worked on "Go to Mat" for Sassy Pants and "Right" (vs left or shake) for Grouchy Pants.  He still doesn't quite get it yet.
We all went for a walk on the beach today.  We had a couple pass by close, Grouchy Pants did ok.  Then we had an unleashed, unsupervised dog run towards us and greet us.   Grouchy Pants was ok but there was some growling.  I think he now has leash frustration.  Sassy Pants and I lured the dog away, we saw it come from one of the houses on the beach, but he ran after us again after we had passed his house, Grouchy Pants was WAY growly and this time the dog got the message and went away on his own.  I think Sassy Pants and I were too tempting for him, we were clearly friendly and most likely fun so he wanted to give us a second try but Grouchy Pants told him off.
Now, there is a leash law and while I admit if no one is around, I'll let the dogs off to run, I always leash them when someone/thing shows up and especially if there is another dog around, especially a leashed dog.  Why can't other owners be more responsible?  I have been guilty of letting Grouchy Pants get loose once, when I didn't realize a door was open, so everyone is allowed some mistakes, but I think some do it knowingly...