Spare us the bleeding hearts for Delta and Continental
Yes, the Shannon stoppover rule is ridiculous, and is a relic from an long-gone era but nobody is forcing US carriers to fly to Eire. If they choose to do so they are bound by the same rules as everyone else operating Trans-Atlantic routes from Ireland whatever the Nationality the airline is.
The carrier most affected by this is Aer Lingus where the rule affects 100% of their long-haul operation.
As to your amazement that US carreirs "tolerate" this policy, the rule was in force long before Delta and Continental flew to Ireland. When they applied for licences to serve Ireland, they knew their flights would have to operate in line with the Shannon rules, and they commenced service under those rules.
This is all political and as it appears that retaining the rule could help win an election, then it will stay, however much airlines moan or even withdraw services.
Again. I'm not saying I agree with the rule at all, I don't - but the "outcry" at what Delta is being subjected to by the cruel Irish Gov't is a bit OTT, now if Aer Lingus were allowed to fly non-stop and all US carriers had to stop in Shannon, then I could understand it!