The Dropkick Murphy's are incensed that Scott Walker is using their "Shipping Up To Boston" as an entrance song. They hate him because, well, they're from Quincy, and they're Irish, and being a pro-union Democrat is simply in their blood.
I have written many songs, none memorable, and have a few other creative endeavors over the years, and I absolutely get how unfair it would feel for any of that were used in support of a cause I disagreed with. Unfortunately for them, unions are just about the one group in the country that would be hypocrites to object to that. It was a specific strategy in the 20th C for union organisers, taught to them by the CPUSA, to repurpose hymns, popular songs, and folk songs with new lyrics. You will find a couple on just about every Pete Seeger album.
I don't want to therefore call it a communist strategy - I think the CPUSA got it from 19th C nationalist and revolutionary groups, which in turn may have gotten it from Protestant revival groups going back to Luther, at least. It is used because it is effective.