That is a good question.
We know in the real world of justice, adopting a “zero tolerance” stance, and a “guilty until proven innocent” puts the accused at a great disadvantage, and runs a risk of convicting innocent people. Part of the balance what is the trade-off between convicting innocent athletes to catch a few more guilty ones, or protecting innocent athletes at the expense of letting some guilty ones go free.
One problem today, is that the current system gives the same penalty to CJ Hunter (2000 ng/ml) as Houlihan (5.2 and 5.8 ng/ml) as to any athlete who accidentally ingests nandrolone through, say for example, eating wild boar sausage.
One partial solution, which seems like it should have been applied here with the new TD (and at least one CAS panelist agreed, while the majority disagreed) is to consider the result an ATF, and not an AAF, and require more testing.
Another improvement would be to consider reasonable doubt, given that all parties agreed on oral ingestion, and the lack of a pattern, and the low quantities involved.
But as the case of Simon Getzmann shows, an innocent athlete being able to prove it under the current system depends a lot on luck, and even then, winning comes at great personal expense and the loss of 1-year of competition.