The black market refiners continue to thrive. Shell can't stop it, so it puts up with it. It's a cost of doing business in a cr_ppy part of the world where corruption and criminality is rampant.
I don't excuse Shell for their mess in Nigeria, but also realize that they have to deal with deep-seated corruption there among politicians and anyone in a position of power. Here is another take:
No environmental enforcement, greatly underpaid officials, underfunded police.....Living with little government is always much worse than living with lots of government.