1. Barry Bonds has not been proven (by a court of law, by MLB's internal testing, or by his own admission) to have taken steroids or any other performance-enhancing drugs.
2. Even if he did take them (which, sure, in my completely uninformed and without admissibility to any governing or regulating body opinion, he probably did), they were not against the rules of Major League Baseball for the majority of his career, when he hit the vast majority of his home runs.
3. Despite the fact that Bonds, if he did use performance-enhancing drugs, is hardly the only MLB hitter to do so, he is the only one to have come even remotely close to breaking the all-time home run record.
So what's the big deal?
Even discounting the ready availability of steroids, HGH and the like, Bonds played in the era of the bandbox ballpark, the expansion-diluted pitching staff, and the (allegedly) juiced ball, but he's the only hitter who takes this kind of shit. Every hitter who ever played for the Colorado Rockies ought to have an asterisk next to their name, but they won't get one. I don't remember this kind of bullshit in '98 when Sammy Sosa and Mark McGwire were vying for the single-season home run crown; far from tainting the purity of the game, they were praised as the saviors of baseball. And yet everyone 'knew' (as much as they 'know' about Bonds), even then, that McGwire and Sosa were probably juicing. And today, when there's little doubt -- McGwire is damned by his own hand and the testimony of others, and Sosa's catastrophic collapse and suspicious taste in friends has created more than enough reasonable doubt -- they don't catch nearly the heat that Bonds does, despite having broken another supposedly sacrosanct record. Bonds is and was a better player than both of them; McGwire was always a one-dimensional crusher and Sosa's home run hitting came at a time when every other aspect of his game was on the decline, whereas Bonds was always great, a terrific runner and a canny slugger, a fantastic fielder and a smart hitter who knew what swing to use when. He won a stunning three MVP awards before he became known as a home run hitter.
It's enough to make you think that his real sins are things like hating the press, being short with the fans, and failing to be properly deferent and respectful. None of which, last time I checked, earn you an asterisk.