Now, the explanation:
Chapter I: The Media
All the comments, complaints and the march are only happening due to a decision done jointly by the british and brazilian medias, both deciding to give full coverage to the case.
This means that, if this death just received the average treatment done to similar cases, there could be a normal trial, indemnization to his family and so on, but not the "media show" over it, and the march.
Why the Media decided this way? Because what the news channels just want to talk about is... TERRORISM! Just because of that.
Chapter II: The Brazilian Inferiority Complex (Average Brazilian Behavior Lesson to Galileo)
The average brazilian (AB) has an inferiority complex. This complex come from the not wealthy status of the country and from the strong belief, or a strange necessity to believe, that life is unfair for him and that his situation is due to prejudice he suffers (for being black, or being poor, or being handicapped... or, in a global basis... for being brazilian!).
As a result, in order to not get killed by this complex, the AB tries to "compensate" it by two ways.
The first one is called SOCCER (tipical thinking of the AB: "We don't live well but at soccer nobody beats us").
The second one is trying to appear or to see other brazilians appearing in the International Media, as a way to think: "Hey! We exist! We are important, too!".
Appearing in a good manner? Not necessarily! It can be in a "good" situation (like Winning World Cups or Carnival Coverage) or in bad ones, provided that the brazilian is said to be suffering some kind of prejudice ("That's why we are not developed! We have nothing to do with it! It's because we suffer prejudice! See? The english cops killed a brazilian!", they think).
About watching other brazilians, our TV Channels are constantly trying to find a brazilian, for more difficult it can be in some occasions, to show to the audience. Sentences like: "You see, the sister-in-law of the cousin of the mother-in-law of this actress that is winning the Best-Actress-in a Role Oscar is brazilian" are, unfortunately, VERY COMMONLY repeatedly every year in the brazilian Oscar coverage.
In this context, it's strange to say but, in a situation like this one of the death of Jean, even though people is sad about him, there is a hidden pride of Brazil being remembered, being on the top of the news for some time... See Blair talking about a brazilian? PRICELESS, then! ("Brazil exists, and we are suffering prejudice!", they think).
Finally, about appearing... well... for the smallest oportunity groups of ABs have to give a message that they suffer prejudice or that they exist... THEY WILL DO IT.
So, since this case got notorious, the "smallest oportunity" stage was OK. "If we make a march to the Scotland Yard, it will go on TV", is the idea. Let's simply complain about the way he was killed, ask for different procedures and/or ask for trials against the London Police Dept? Of course not! We must cover ourselves with brazilian flags and sing the hymn (the same hymn the AB DOESN'T KNOW OR BARELY KNOWS), because we will appear! We will exist as brazilians! We will... EXIST!
And, the "best"... we will appear as... VICTIMS OF PREJUDICE! "Good", isn't it?
Other oportunities to appear as brazilians come in sports events... even our soccer players, already known in the whole world and supposedly used to the Sports Media, can't resist insisting on non-verbally stating they are brazilian and that "Brazil exists" by dancing a shameful dance we have called "samba" during the prize ceremony, just after the final match, in front of soccer autorities... they can't just smile, kiss people, cry and celebrate in the hotel... they NEED to state, as if there was a single soul in the stadium or in front of the TV sets world who didn't know already, that they are brazilians...
Chapter III: The Anti-Cops and Anti-Resolution of the Problems Brazilian Culture
The brazilian culture is against any efficient way to solve of any problem, included the one of criminality.
Because of this tradition, the maximum possible sentence in Brazil is the one of 30 years (we don't have prison for life here... if someone kills 15 little girls... the maximum punishment this person can have is... 30 years) and whoever goes to prision or commits a crime is, even before being caught or even recognized, is generically seen as "a good person suffering prejudice from the SYSTEM" and... automatically... is WELL SEEN by the ABs! If the criminal is seen crying at the Media, "OH!"... with a little luck, the ABs wants him free right away!
An example of this behaviour is the Ronald Biggs' one... he was one of those who did assault the cash train in London and injured the train driver for life and got to Brazil, living in Rio de Janeiro City... and he was HONORED in our Carnival!
So, as the "other side" of Jean's death are COPS, more reason then to the ABs complain in the streets!