If the UK doesn't put up a border then it is the EU that is causing a problem. There doesn't have to be a hard border, but EU insists that there should be.
The only reason they voted for Brexit was because they think it means they can get rid of all the migrants, they tell me this quite regularly.
One of them moved here from Bahrain, though apparently this means their family were ex-pats, not migrants. The other is from a long line of Christian overseas missionaries.
If they thought for a second that Brexit would mean leaving the border with Northern Ireland wide open, they would be screaming blue murder.
The UK is leaving the EU, borders are implicit in that, and bi-directional. You don't get to leave the EU and not leave the EU.
Can you cite the Poland/Russia thing? I searched but couldn't find anything. edit You referenced https://en.wikipedia.org/wiki/Poland%E2%80%93Russia_border in another post.
> Because Kaliningrad Oblast is small, homogeneous and an exclave within the European Union
That's not the same as the NI situation.
Regardless, the EU is negotiating in it's best interests, as anyone could predict it would. If I were French or German or whatever I'd want to get the best deal possible in the negotiations. No-one is going to give us any favours here, everything we want we've got to make happen ourselves. If the EU is using the Irish border as a bargaining position then that's what they should be doing, in the best interests of the citizens remaining in the EU.