Another Look at the Initial Issue
Do we have an infringement issue here? Likelihood of confusion might be argued, although the band's mark is more elaborate, and it isn't necessarily hawking clothing, jewelry, perfume, accessories, etc. (yet), or targeting the same customer as YSL. The YSL mark, however, may very well qualify as famous, obviating the issue of categories of goods or services and leading to a claim of dilution.
Still, Yeasayer's YSR may be sufficiently distinguishable enough from YSL to make naysayers of the legal doomsayers.