Well, we're dealing with 2 different issues here.
On the one hand Viht sell bulk powder to the trade and public. That's their perfect right. They sell all over the world. Not every country shares our philosophy on H+S matters.
I know of one dealer who decants from bulk into 'portions'. That's his right too. He will most likely have bulk handling facilities. That goes on with all sorts of chemicals etc etc.
On the other, well ,you've got HSE, DoT,Explosives regulations etc etc. all designed to control the safe handling, transport and use of powders and primers etc.
The 2 are opposed to each other, but generally work out o.k.
I wouldn't feel very comfortable topping up my trickier from a great big drum, would you?
After all, you can buy 50 gallons of petrol if you want, but you can only store a , relatively, tiny amount in your garage at home.