This is just a stab in the dark but if the OP’s friends were sitting in a “souped up” BMW, that sounds like it might be traffic cops rather than just “normal” polis, doesn’t it? So what you’d have is an example of traffic cops enforcing traffic law. And they didn’t ticket him or give him producers, they just told him to wear the fing seatbelt. Sounds like OP copped a massive attitude and got all Barrack Traffic Lawyer about it too.
I usually criticize the police and the job they do but in this case I don’t see the problem at all.
By the way, the lack of a seat belt serves as a pretext to interact with you, see if you seem pissed, have a quick look at the car and tax disc, maybe ask for licence etc.
And I bet everyone that gets stopped for not having a seat belt says “oh ffs what are you whining about, I was just about to do it up, 10 seconds won’t kill me…”