Clenbuterol is prohibited by the EU in food-producing animals. It is (AFAIK) not approved for any therapeutic use in humans either within the EU. Btw. before you complain about my Eurocentric views, both statements hold true for a much wider range of countries, but I'm too lazy to research the precise extend.
Anyway, the only way to get clen into your system (living in countries for which the two above rules apply) are by illegal use (either in the food industry, or your own illegal use of prescription medication).
Now I can understand that you might balk at strict liability when it concerns illegal use in the food industry. I would contest your assertion that it is impossible to show when that happens. For starters, it's never just one cattle which is contaminated, usually these kinds of manipulation are done to a herd at once, so there's usually more evidence than just one single steak. Second, there's always parts of the cattle which won't be consumed at once. Parts of the cattle will find their way to a lot of other products and those pieces might be available for testing. Third, there's the hair test of the athlete. And I could go on with plenty of other things which could be done if the will is there. My understanding is that none of that was ever even attempted. It was portrayed from the beginning that it's virtually impossible to show accidental ingestion of clen. This is not true. It's a smokescreen set up for the benefit of one person.