As far as I know it is because the products have to be sold with a document that certifies their safety/authenticity etc. for the US Customs/Dept. of Agriculture . Only the "Wrights of Howth" shops in Dublin Airport give out these certs that I am aware of - I dont' think any non airport retailers issue these certs .
Apparently the shops will sell you the product but you are not allowed to bring any meat or meat products, or dairy products into the U.S. even with a certificate. A friend of mine got caught recently with the good old rashers and sausages by US Customs in JFK. They confiscated them from him and fined him to boot! (Probably had a good fry-up themselves!) When he came back he checked with the American Embassy and found out that US Customs were correct and he wasn't. So out of pocket on two counts!