http://www.bbc.co.uk/news/uk-politics-42231497
"it would prevent Northern Ireland from leaving the EU "on the same terms" as the rest of the UK. Northern Ireland would effectively have remained in the EU's customs union and single market in all but name."
"Scotland's First Minister Nicola Sturgeon said that if Northern Ireland was allowed to operate under different rules there was "surely no good practical reason" why other parts of the UK could not do the same - a message echoed by Welsh First Minister Carwyn Jones and London Mayor Sadiq Khan."
"Ireland's Prime Minister Leo Varadkar is understood to have insisted on the clause"
Ah, and there was I thinking that an IRA member had to have been the one making the suggestion to split up the UK and unite Ireland. Turns out it was Leo cynically chancing it under the ridiculous guise of creating a soft border.
The UK has already suggested a better, and probably the only, solution with an open border, as today, but with technology monitoring. But those who only use the issue for their own agendas don't like one that doesn't progress what they really want.