Of course, the parks cannot be kept open for other people, because the police and council officials could hardly be expected to stop people entering "on suspicion of being Iranian".
I think Londonist strikes the correct tone:
It is true that Iranians don't have a very good grasp of the Keep Britain Tidy business, but this really is quite Scrooge-like on the part of Barnet.The Iranian community has tried to negotiate with the council and police to facilitate this festival but the council just appears not to have the patience to find a way through. Much simpler just to piss on everyone's parade.
The Barnet Press reports the decision as well - see here. They get it slightly wrong, though, in that most of those wanting to picnic this weekend are no longer Iranian but British nationals.