So everybody who trades with a sanctioned country is itself sanctioned? Like a Sanction Midas Touch?
For better or worse, the US Banking system handles pretty much the world's banking - it is a stable system, both in terms of system uptime, and in that no government minister is just going to funnel it into their own pockets with no notice. Losing access to that system means you have to bank like its 1950, only without the benefit of everyone else being at 1950's levels of banking.
It is basically like living in a city and the water company blocking your access to water & sewage - You are now reduced to buying pallets of Evian and shitting in a camping toilet. Your neighbors won't even let you use their shower or fill up your cistern from their hose because they know if they get caught, they're going to be shut off and put in the same boat.
If you are caught handling transactions on the financial network for a barred entity, you are now also barred. Anyone who did transactions with you is also subject to scrutiny and a potential barring as well. Regaining access ranges from 'paying a fine and saying you're sorry' to 'a fine and auditors (that you pay for) all the way up your ass until they see your molars' to 'hahah no get wrecked'.
So since Pvdsa was barred, that means that Russian oil company can still do business with them AND keep banking network access, but they have to make sure those streams of income remain wholly and completely separate - you can't even allow for even the
appearance that Pvdsa is being sent money from the network or being allow to send money to the network. It is much, much safer to just say "Nope, fuck off with your Zimbabwe dollars", especially given the tensions with US and Russia over this (And syria. And ukraine).
