The problem is that Hamburg is the Antifa and Autonomism capital of Germany and they really, really don't like the G20 (=antifa in this text for the violent parts of it). They see the fact that the summit will be held in Hamburg as an open challenge from the German government. The politicians obviously have nothing to fear because they will be protected by the police. The 20.000 police officers however who are out there in Hamburg to protect the summit will be the ones who have to deal with all this shit. Of course the Antifa would have come no matter where the meeting would have been, but choosing Hamburg will most likely increase the level of violence.
Btw as I understand it the U.S. are quite new to the concept of Antifa and their version seems to be different from ours because it is often associated with the democratic party or liberalism in American media. The German (and european) antifa is not liberal. What exactly it is they are I can not tell you, I would say it is a mix of Anarchism, Socialism and Communism. Over all they dislike pretty mutch any government, wether it is Germany, Turkey, Russia, Saudi Arabia, the U.S. or China. Here is a video from them explaining why they are against the G20 (subtitles can be turned on bottom right courner) Example from the video as to why the dislike the governments of some of the G20:
There are really not many political groups they like. They do however cooperate with certain groups during certain actions. For example they work together with Jusos to demonstrate against Nazis (Jusos=social democratic youth orginisation belongs to Germany's second biggest party the SPD). This btw is the reason as to why the word Antifa is not really as clearly defined as one may thing, because groups like the Jusos also take part in antifacist actions, but without violence. Some of the Antifa also work with the YPG and PKK to fight against ISIS, sometimes by flying down to Syria and actively taking up arms. (Example 1;Example 2. The dude in the video is dead now btw.) Something else that distingushes the German and American Antifa at least as far as I can tell is the level of organisation and the amount of people ready to use violence.the video linked above wrote:[...]Germany starves the south of Europe with force austerity, Turkey arrests and executes thousands, in Mexico people are disapearing without at trace and the US has exhibits the worst power elites ever, Russia keeps getting more and more authoritarian and China leaves us speechless. We know of no government we would support[...]
Another problem is that not only the actually politically motivated Antifa from all over Europe will be in Hamburg, but because of all the very well made promotion for the riots that will be going on, there will be a lot of riot tourism. (examples for the promotions: Example 1. ZSK & Swiss; Example 2. Welcome to the Hell: Hamburg 2017.)
So where does this leave us? With a lot of stupidy and a lot of violence. It already started out with burning police cars and sabotaged train tracks.
Just to make this clear, I am blaming the violence on those who commit to it, but I still think it was a horriable decision to chose Hamburg as the city for the summit. It is a bit like organising a football game and only inviting hooligans.
Edit: Nevermind apprently the police are also getting in the mood: Berlin police fired from G20 preparations over ‘public sex and urinating on fences’
It includes hotels for the delegations, police officers, the press, the telecommunication infrastructure, and the complete traffic system. 
