Because NI must never diverge from the rest of the UK in regulations.While the rest of the UK would have to renegotiate it's trade relationship with Europe, N.Ireland would have the best of both worlds ,that means by relocating to N.I. British business could avail themselves of the same privileges.
So logic would dictate that the DUP should support Mays deal ,so why do they not.
Oh - except for abortion of course.
And same-sex marriage.
DUP Manifesto = the Old Testament with fortnightly bin collections.
