Imposing such duties onto airports will be expensive. Smaller airports in Britain are already running at a tiny fraction of their capacity, with only London, Manchester and Edinburgh operating any serious services. A particularly striking example is Cardiff Airport: it can accommodate A380s, but you'll have to be patient if you want to spot a widebody airliner there. It's not really worth the extra equipment and staff to search for money laundering in these quieter airports, and the costs would probably force them to shut down entirely if not funded by the taxpayer.
This situation of reduced capacity is not entirely theoretical, either. Before boarding Eurostar railway services through the Channel Tunnel, passengers have to go through security which includes passport control, customs and X-Ray (for luggage) and millimetre band scanners (for passengers). The introduction of these checks reduced capacity by a third, meaning that the Eurostar trains run with empty seats, when other trains running domestic services on the very same line can be crowded. It's about to get even worse now that Britain and the EU are introducing incompatible facial recognition systems that gate the entrances.
As you can tell, I'm rather passionate about this issue :) I just think it's important that borders don't become the de-facto location on which all types of law enforcement converge, and for border security to be so entirely out of proportion with domestic policing.
