Is this due to the Irish government which is trying to increase tourism in Shannon by forcing trans-continental flights to go via Shannon or is there some other reason?

It’s an agreement that’s been in place long before DUB had direct unilateral links to the USA if I’m not mistaken? I must finish reading my recently purchased book about the subject. A political/business agreement designed to protect jobs and services at Shannon ???

The agreement in Ireland was similar to that, that was in place at Prestwick, pre-early 90’s, when all transatlantic business out of Scotland had to use PIK as it’s point of departure.

That particular regime was made a mockery of (and subsequently abandoned) when Air 2000 famously offered flights from GLA to the USA complete with a brief touchdown at PIK, before heading out over the Atlantic to Florida.