Police is the key word there. It will be nothing to do with Fisher if they get bans or not.
Why are people so surprised? There were plenty of posts on here prior to the match saying if people went on the pitch they risked getting a ban, now its like its shocking unexpected news.
You would have to ask the relevant authorities but I suspect, if you got an answer, it would be something along the lines of it being an end of the season celebration so discretion being used. Same as nothing happens when clubs get promoted and fans go on the pitch to celebrate. But again that is down to the authorities and not the club in question.