Because NI is getting it for free and the rest of the UK would have to pay for the same deal (with or without FoM).I suggest the whole UK signing up to the deal Johnson says is a great deal for N. ireland. We leave as one UK with the same deal as we were told we would and should as the Tories promised the N. Irish we would. If it’s a great deal for N.Ireland and Boris says it is then why is it not a great deal for the rest of the UK, and prevent a customs border within the UK at the same time.
(Cue: but ... Tiger Woods wouldn't have to pay...)