I think that's a British English convention mostly. Americans say "is" in these cases far more oftenI'm American, and it is objectively wrong and bad sounding to say "is" when referring to a group of people. Grammatically, all band names function the same. Saying "Nirvana is" is exactly the same as saying "The Beatles is". Just like team names. Nobody would say "the Thunder is" or "the Heat is".