Why should they have done that?

Their carry-on baggage will have cleared Customs at DUB (on arrival in the EU).

On landing at LHR they will, presumably, have ended up in a common baggage reclaim area after bypassing Immigration (I've never flown into LHR on a domestic or CTA flight, but that's how domestic/CTA arrivals are handled at other UK airports). If my assumption is correct then they should simply have picked up their bags and gone through the green channel.

As we know (but visitors from North America may not) there is no explicit customs clearance in EU countries. You don't have to fill in a form or stop to talk to a Customs Officer. Unless you have something to declare (in which case you have to go through the red channel) you just walk through the green channel and carry on walking unless you are stopped. The green channel is for those who started their journey outside the EU and who are travelling with checked baggage (which is generally always checked through to the final destination). The blue channel is for those whose journey started within the EU, and for those who have travelled via another EU airport with hand baggage only.