Just because they allow it doesn't mean you should though. There's no laws against farting in a swimming pool, but it's still a crappy thing to do. A public park is dog friendly, but it's manners to put your dog on the lead when someone else is passing. I'm not saying they should be on a lead *all* the time, but when you are passing someone else/another dog, then I feel it is the right thing to do unless both parties are OK with the situation. Sometime this is obvious and the other person is clearly just as happy as you for the interaction, but most of the time it's very clear people are not and people ignore this.
If you let your off lead dog run up to a person, child or on-lead dog without permission, you are an asshole. I don't care how "friendly" you think your dog is.