This is a conflation of issues of race and class. What people think are issues of race are often in fact issues of class. A few examples:
Many BAME people that I have met don't listen to music that includes swearing of any kind, and indeed disapprove of it. Similarly, in the Caribbean most middle class people that I met (I mean middle class BAME people) disapproved of the Rasta culture. These are class and religious issues. Class issues between BAME people are just as alive and well as those between Caucasian people. However, the BIG danger of lumping all BAME people together under that one heading is that no account is made for class, which is far more important than race actually in my experience.
So, what I suspect you have here is an ancient trope, namely Caucasian middle class people appreciating an art form mostly created by BAME working class people and therefore ending up mouthing stuff that to them sounds good but that they really don't understand.
In the 1950s most Caucasian people had no idea what the lyrics of "Back Door Man", "Shake Rattle & Roll" & "Hound Dog" really meant. All were created by BAME writers and then sung by Caucasian singers. Similarly, Caucasian fans of Rap culture will happily chant along to mostly USA created texts which, outside of the harsh & brutal conditions in USA inner cities, are in fact texts removed from their original social context.
Does any of this matter? Having worked in schools in inner London, my solution when working with students is the "Granny test". They can listen to whatever they like, but when they create a track for (say) GCSE Music then my advice is "if you wouldn't play it to your granny, don't submit it for GCSE - or at the very least bleep all the Ns, MFs, Fs etc or erase them to leave a short silence". That way they can express themselves in their chosen style but they also experience the same justifiable censorship that would apply to, for example, a mainstream daytime radio station. This is why many artists produce 2 versions of a track, the original and then a 'clean' one for radio play.
I personally wouldn't ever chant along to that stuff for the same reason that if a track was full of the F word I wouldn't either, namely because it's highly likely to offend someone within earshot. We all censor ourselves in public all the time in order not to gratuitously offend others, at least I hope so.
Your friends have taken on second hand a culture that has been 'beamed in' from another place and context. The REALLY interesting question here is this - why does that music and the attitudes expressed clearly speak to them more than music created in the UK? Maybe it's viewed as a bit 'naughty', maybe they think it's exotic...or maybe they just don't understand the social context that created it.
Being optimistic for a moment, Caucasian musicians misunderstanding the Blues gave us Rock, Metal etc. Right now, there is a similar misunderstanding taking place, which may in the future produce something quite interesting.
Finally, don't forget that in fact the bands that created R&B, Motown, Stax soul etc (the writers and session musicians) were all a mix of races. Musicians are the least racist people I know, and that makes me proud to be one. Musicians resisted segregated audiences in the USA, refused to play in Apartheid South Africa etc. When musicians work together, the only issue is - can you play/sing/write? Long may that continue!