Curtains in lorry windows

I know we have some drivers out there that could perhaps throw some light on the subject for me....What is it with curtains in lorry windows???

I see a lot of wagons round here working close to the docks and there seem to be an increasing number with fancy tasseled curtains up at the side windows and even pemets across the front.

My route to work brings me onto one of the main routes to the dock from the motorway and I approch from a side road at one of the roundabouts. I often feel unsafe as I can see and increasing number of wagons that cannot see out of their side windows due to these big curtains (or at least they have to lean right forwards to peer round them, like the guy who only spotted me at the last minute this morning!).

Why is this not ilegal? (is it?)

You would be stopped for putting heavy tints on the side windows of your car, surely curtains on a wagon are just as unsafe?