Because the rule is quite clear and based on the that 24 hour time frame.
Self connect after 24 hours and no problem .Self connect or travel on the same ticket within 24 hours and the duty is Payable period and if you don’t that is evasion not avoidance which is a crime petty in know but still.
ISAs are hardly an analogous, Those are saving schemes specifically designed to encourage smaller saving investments from earnings already taxed elsewhere.
All that said the actual numbers deliberately using the self connect options particularly via Dublin are again probably rather smaller than representative within the aviation interests section we subscribe to here and in the greater scheme of things
Much of the traveling public live blissfully unaware lives and such things particularly irregular travellers whom are likely to visit either the consolidators or airlines directly will pay the taxes anyway.
Repeat and rewind Dublin is an growing connection point for the north altantic - total transfers a little over a million a year significant amouts of which are from or to mainland Europe.