The MMM Act is quite clear in this aspect. You are not protected for the BURNING OF CANNABIS in a Park.
A park is a general recreational place officially recognized in any given County or City. I have never looked at the actual laws around this, but all "Parks" would be City Owned, thus community properties. Same thing would essentially apply to any government owned area with no official duty other than that of a recreational nature. like a Fairgrounds or swimming area (usually in a Park) unless privately owned.
Kind of ironic, Recreational Pot will not be useable in a place deemed as official government recreational properties. lol
I ll have to re read that section, but my interpretation of the No Park, Buss piece deals with the ingestion of Medical Cannabis as otherwise allowed under USE.