That they were already Hall of Fame guys before the juice. The vote totals compared to guys like McGwire and Sosa basically bear that out, because most people feel the opposite about the latter 2: Juicers for virtually all of the elite portion of their careers.
I personally just don't like voters trying to parse it that way, because we don't have the real information, and it takes a lot of assumptions to make that decision. Either you accept that the juice was unfortunately part of the game and judge everyone against the contemporaries of their era, or you penalize all the known juicers, regardless of where and when it's believed they used.