To help prevent DVT all you have to do is drink plenty of non alcoholic or non caffeinated drinks and once every 30 minutes or so go for a walk.....However,when seated ensure that your seat belt is loosely fastened to guard against any sudden turbulence.
If the seat belt sign is on then fasten your seat belt....it's in all the PA's and safety cards...Not really rocket science yet how many people get caught out.The cabin crew are doing their job and one in which has a certain amount of inherent danger which cannot be helped but minimised by not working during times when the seat belt sign is on.
If it is neccessary to have the seat belt sign on for pax safety then it is also required for cabin crew.Their necks and limbs are not any different from the pax.
The problem is that common sense is not common at all and that most pax do not have any idea of the environment in which they are travelling and the potential for incidents.