> general lyrics about illegality are just general lyrics whereas specific lyrics, with other evidence could form the basis of a case.

IANAL but that is the case. They aren't trying anyone for the lyrics. They're trying them for a crime, with lyrics that suggest an admission to a crime serving as evidence. As far as I know no one has been sent to jail simply for the lyrics. However, it is quite convincing to a jury when you are on trial for a crime, and you also rap about said crime brazenly. No different than people going on Facebook to brag about what they stole from the store.