Why did they use ultrafast diodes in a 50 or 60 Hz bridge?
As well as possibly rationalizing their BOM (though I would expect Schottky diodes to be used on the low voltage side rather than fast recovery conventional diodes), they would possibly have had to put a high voltage capacitor across each of the 1N540x diodes to reduce EMI (from the glacial reverse recovery times- unspecified generally, but in the microseconds- and subsequent fast snap-off), and the ultra-fast diodes don't require that. 

Here is a paper from Power Integrations which covers the comparison, from which the below graph was drawn: 



Interestingly, the author says that it is sufficient that two of the diodes are fast-recovery. 

This is unrelated to the switching regulator- you'll see this topology (bridge with four capacitors) used in transformer linear supplies used in audio equipment to prevent annoying 100/120Hz buzz.