I'm sure it's just as clear to you, as it is to anyone else, who hasn't really given the issue a second thought.
Also, borders exist between two nations, so it's up to both to determine their nature -- unless one unilaterally decides to close it. But as long as we are insistent on an open border then either, by definition, the rules on both sides ought to match exactly. That is the issue, and you can't say that the EU haven't made it clear while in the very next breath spelling out yourself what their problem is.
Funnily enough, it's been suggested in various media outlets that David Davis came up with a wonderful solution*: namely, that Northern Ireland could be granted joint EU/UK membership. If only we'd thought of this earlier!
*Subsequently somewhat denied, but not flatly ruled out, so it's likely that the claim in the Sun was not wholly unsubstantiated.