Ah, the good old posthumous single. 2Pac has had more successful releases since his death than during his lifetime. Every "previously unheard" Nirvana song is instantly snapped up by fans of the Seattle trio. Elvis got to number one a couple of years back despite kicking the bucket in 1977. And even the 'King of Reggae' is getting in on the act.
The problem is that it doesn't always work. 2Pac's material consisted of lengthy tapes full of acapella rapping. It's easy to put a hip-hop beat under that. Kurt Cobain's lyrics were accompanied by some ideas for guitar tabs. But all that existed of 'Slogans' was a demo vocal.
So when his sons Stephen and Ziggy decided to turn that vocal into a single, they needed to come up with an entire piece of music. And in their desperation to produce a song that sounded like Marley's other work, they've essentially copied the music to 'No Woman No Cry'. It's like a bad cover version, with two redeeming factors - the vocals (which are unquestionably Marley at his finest), and the appearance of Eric Clapton on the guitar part.
But in fairness, no-one can have any idea if 'Slogans' would have actually sounded anything like this. It'll definitely help sell more copies of the new singles collection though!