Well, in that era, to have a chance of being competitive, you had to dope
I know, but Jan's dreadful (denial phase) argument logically implied he was only attempting to dope exactly the same amount as his rivals. He wanted it to be "fair". That means he never took more, or better, "dope" than his rivals..just the precise amount needed for racing to be perfectly fair.
Or..as we all know...he did his utmost to take more, or better, "dope", in order to gain any advantage he could, to make the playing field as sloping as possible, in his favour.
Everyone knows they cheated..it would be nice if they all manned-up and said "yes, I cheated". It doesn't need a "...because...", we know why, it was for money and fame.