A Brexit question Boris Johnson must answer

The question for Boris Johnson is why he signed a Withdrawal Agreement that gives the EU the power to decide whether British agricultural products are fit for export to Northern Ireland. There was no secret that this is in the Withdrawal Agreement. It is there in black and white. Critics of the deal cited it from the start. It is why more sanitary and phytosanitary checks will be required on livestock and agrifoods going from Great Britain to Northern Ireland. 

So why would Boris Johnson sign a deal knowing from the outset that its terms were such that in plausible circumstances they might lead him to breach international law? Or was he was unusual in simply not knowing? 

As for the EU’s take, an official says: ‘We still need to list the UK as a country that can export agriculture stuff. It would help if the UK could explain its future regime on food production standards,hygiene, etc.