The rationale is that you need to prove your eligibility to rely on the CTA, and the best way of doing this is by presenting your passport to show yourself as a UK or Irish national.

Although there is no border control, you do need to show eligibility to use the CTA. I have been on the express bus from Belfast to Dublin when the Guards have pulled the bus over to check papers.

The truth is more that DAA just couldn't be chewed making separate UK and international arrivals, I think.