What you typed is what they do, but there shouldn't be any extra visible exhaust emissions during a regeneration cycle, sure the carbon is still being released, it has to be, but afaik, they work by recycling a portion of the exhaust gasses in such a way that they are used to heat something which literally burns the carbon rather than it simply being emitted as in the more usual cloud of black soot you see coming from buses and non filtered diesel cars.
Clearly it must still enter the atmosphere, so it doesn't help in that respect, but (without knowing all of the details), it is better for it to be done like this than allow it to be pumped into the atmosphere in the form it is created from unclean burning of diesel fuel. Specifically, I imagine it leads to cleaner air and less of the black soot you get covering everything if you happen to live in a city...
The cars you have seen have just been idling for a while, don't have filters, and the driver has just booted it. Or perhaps they are massively over-fuelled (read: badly chipped/tuned...) . Nothing worse than a highly/badly tuned VAG diesel tbqh, the amount of smoke you see coming from cheaply chipped golf's etc. is astonishing.
having said all that, this is all knowledge based on my research of the FAP (*sigh*) system, Fiats DPF may be different!