The thing that gets me about a lot of the complaints is that 2 million visitors is the equivalent to a very large UK city (I think there are only 3 that are larger), and most of them are going to be exactly the sort of people that criminals traditionally look out for regardless of what's going on, namely people out to have fun and who are likely unfamiliar with the area.
Pretty much every carnival or major event has a "higher than average" level of policing and crime because the criminals know there are going to be a lot of potential victims and cover for what they're doing, whilst the police know that there are going to be those criminals plus all the normal issues you get when you've got loads of people together, but unlike most events this is massively spread out and not largely contained in an area that has been at least somewhat converted to it (unlike say most big sports arenas which these days are typically built with crowd control and crime levels in mind).