U2's whole purpose in the 90s was showing how ridiculous it is to be a rock star. How ridiculous it is that just because they happen to write music that people like, they are treated like Gods. And how ridiculous it is that their identities are sold exactly the same way food and clothes are sold. How ridiculous it is that our society(and every developed society in the world for that matter) allows massive wealth for 'rock stars' and 'movie stars' and 'tv stars' - because 'rock music' and 'movies' and 'tv shows' SELL in places like K-MART - while there are people doing more important things but making far less money because they DON'T SELL. Popmart was ultimately a critique of the ridiculous importance society puts on money, fame, and image, and how celeberties all ultimately become just another item on a shelf in a store.
The K-Mart press conference was essentially U2 selling THEMSELVES as living, breathing merchandise in a bastion of consumerism as a way of overtly and bluntly illustrating the points I made above - the way their craft, and indeed the craft of every 'rock star', 'movie star' and 'tv star', is bought and sold as nothing more than a product.