Evo said:
How can this be unconstitutional if it is criminal?
http://www.boston.com/news/nation/articles/2011/08/09/for_flash_mobsters_crowd_size_a_tempting_cover/?rss_id=Boston.com+--+Latest+news
If you explicitly organize a group with the intention of causing violence, it's surely criminal. The thing is, that's already a crime.
http://www.clevescene.com/scene-and-heard/archives/2011/08/04/mayor-jackson-vetoes-clevelands-flash-mob-law
http://www.clevescene.com/scene-and-heard/archives/2011/07/26/clevelands-flash-mob-law-fuzzy-maybe-illegal
The law, as it's written, adds penalties for not only actions, but how the person found out about the event.
To understand why this is a constitutional issue: Many cities already have laws on the books about "illegal assemblies" that wouldn't past a constitutional review. Same thing with "failure to disperse" ordinances.
Imagine a legitimate political protest where people are arrested for various reasons. Even if the original reason they were arrested turned out to be invalid, they are now facing a second charge they have to defend based on the way they heard about the protest. Furthermore, this is essentially penalizing people not for the action, but for participation in free speech. For example, if someone was arrested at a protest, and they had had heard about it through a flyer, they would be facing lesser charges then if they heard about it over the internet.
These arguments may sound subtle to you, but they are an extremely important topic right now. Since more and more political organizing and communication is done over the internet, the potential chilling of that vehicle is very important to people who are concerned about civil liberties.
Let me give you a practical example. A group is planning an anti-war protest, and they distribute information over the internet. Members might be inclined to say, I am hesitant to go because if an arrest does occur, I will now be facing greater repercussions then before. This is a legal disincentive to use the internet as a means of communication.
It also establishes a bad precedent, targeting methods of speech instead of criminal actions themselves.
EDIT: Furthermore in order to be enforceable (which it probably wouldn't be) the police would have to be able to search all of a persons' communications to have proof they received knowledge of the event electronically, and possibly their friends communications.I am not trying to argue the validity of these positions, I know you and I have a very different take on such issues, i am just answering your question from the point of view of civil libertarians.