Ehhhh......
As a non-smoker (of all the things) and a parent, I'm going to have to disagree. You don't own the public property. The city owns the park and if they don't have a no smoking rule, you are the dick to roll up on someone and tell them leave. As well, you are a dick in general for rolling up on someone engaged in a legal activity and telling them to leave. How entitled is that?
It is legal to smoke weed in a state that has ruled it so. You can have X amount on you. You can smoke it in public, etc.
If I rolled up on the park and there is already someone on a park bench or hell even the swings smoking a cigarette. I would just relocate to another area of the park. That person has just the same right to be there as I do. They don't have less because they are over a certain age or haven't spawned like I have. This goes with all legal substances that can be smoked. They were there first. We are getting far too entitled.
Now, if it was a school (not public property but hey they tend to let you use their play stuff), then I'd be like whoa dawg. Or if I was there first playing with my child and someone rolled in smoking, I'd be like -- um.. dude, since we were already here, can you extinguish that until we leave or take it over there?
I mean.. this is not a hit at anyone, it's just my opinion on it.