Cabin crew are on their feet during taxi for safety related duties, i.e. required duties such as door mode selection, the safety demonstration, and cabin secure checks. Crew are covered by insurance for injuries sustained whilst carrying out these duties.
Passengers do not have any duties to fulfill, nor any valid reasons to be standing while the aircraft is taxiing, and this is taken into account for insurance reasons. If a passenger is injured, or by extension injures other passengers or crew, due to not being appropriately restrained whilst the seatbelt sign is on, they must be held responsible for this and should not be covered by insurance.
I take a common sense approach to this whenever possible. If seatbelt signs are on during turbulence and a customer is in desperate need, I will assess the situation. If I'm standing, I will generally allow the customer to use the lavatory, but only after warning them that it is at their own risk. If I'm seated and harnessed under orders of the Commander, then it is by no means safe to leave your seat.
On the ground, I apply a similar logic. If I'm seated for takeoff, everyone else should be seated too. If I'm in the cabin performing the safety demonstration, everyone needs to be seated. If I'm doing a cabin secure check, same situation.
On most European airlines, the seatbelt signs are on for quite a short time in comparison to US airlines. While not wishing to sound like a "nursery nurse", I should hope the average adult would be able to hold it in for about 20-30 minutes.
13 please has not, from what I can see, shown any contempt for the customer, but has rather commented quite accurately on human nature. If one person gets out of their seat while the seatbelt signs are on, others will follow.
Capot, your arguments concerning aircraft taxi speed are interesting. I would suggest, however, that an airfield is host to a variety of moving machines of varying power and size and it would be sensible to have everyone seated where there is no requirement for them to be standing.