Why at the end of every american voilent/crime film is their a rap song in the credits?

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I just finished watching fast and furious five also known as 'Fast Five', first things first watch it it’s the stuff! Now that we got that out of the way, after I enjoyed watching one of  the best car films ever made the first song in the credits was a rap song called 'How We Roll' by Don Omar featuring legendary rapper Busta Rhymes. Now after this a thought occurred to me why do many American films that are about negative or violent views all have rap films in them? 

This must mean that people tend to associate rap with violent and negative things which must degrade it as a musical genre in society? Then I asked myself can I really blame society for thinking this way? Obviously not I told myself my reason for this is  2 thirds of rap is gangster rap. As a fan of rap and an aspiring rapper myself I have listened to and researched some information about musicians, especially rappers and most especially my favourite rappers. So I have some understanding of how gangster rap actually started and why it was so strongly received; gangster rap was mainly about civilians in gang polluted areas speaking about their experiences that attracted a large audience because some people who couldn't relate were interested and people who could related liked it too. It is similar to the grandfather/father concept telling his grandchildren/children camp-fire stories of their experiences expect for every line rhymed with another, they were delivers a melodic articulation described as flow and they had backing music called a beat. Even though this form of rap started to help a lot of people mentally and physically and it became therapeutic, as every beautiful thing created it got corrupted by liars, money hungry and business people. I will touch on this topic more deeper in a later time.