The English channel and customs controls do not interfere with the right of EU/UK nationals to move freely across the border. If hard borders are not instigated on the NI/ROI border, then there is no physical barrier to prevent free movement.

Therefore, to instigate border restrictions impeding freedom of movement on mainland points of entry are discriminatory. And if a nasty DUP supporting piece of work is entitle to free movement to/from EU (aka ROI) then so is everyone else in the UK.