the reason is this!
when qatar had no street names or accurate maps of anything other than inner Doha people had a problem. Easiest way? meet me at so and so roundabout! Police were relaxed about it, after all they parked their cars their too when arriving to "help" speed up the traffic!
Now things have changed, they have dedicated spots for police cars to park, when oh when will they realise that any block on a roundabout including traffic lights stopping traffic dead within the roundabout, leads to more delay!
Legal to park? I doubt it, but who will enforce it!