Bootlegs are a perennial bone of contention in the music industry and have been since the first significant one - The  Great White Wonder, Bob Dylan's legendary, unreleased  basement tapes from Big Pink, recorded during his year's sabbatical following a motorbike accident - appeared in 1960. Some record companies and artists consider them artistic theft since all profits from their sale go into the pockets of the
bootleggers and none go to the artists.  There is also the issue of quality: most bootlegs are either poorly recorded live tapes or else studio outtakes; by definition sub-standard sonically or artistically, or both. 

The opposing argument is that bootlegs comprise material that the artist would never officially release anyway, so they are not being robbed of royalties in the real sense.  As for the quality: the fans know the material is usually poor, but don't particularly care, they just want everything they can lay their hands on by their favorite act. And to suggest that bootlegs might somehow reduce sales of official material is absurd since those fans keen enough to acquire bootlegs will almost certainly already own every official release.