Um...first of all, Martina has not been proven guilty, and, at least here in the United States, she is therefore innocent and this proves nothing.
That's in the court of law. The WTA= a business. Therefore, they can use whichever standards they deem fit. They could very well assume she's guilty, with the burden of proof placed on her, not them. And they'd be well within their rights to do this. Why should the WTA have to prove to the media and the public, as well as Hingis, that their drug tests were in fact accurate? Do you think any other employer would do the same should a worker come up positive?
Second, even if she were proven guilty, it would be for using a recreational drug, granted a really dangerous one, but still not a performance enhancing drug. Also, Martina's hair test shows that there is not a traceable amount in her system, meaning if she did use it, she used it one time in a small amount (and even that is unlikely.)
Cocaine is a stimulant. It was used for centuries as a medicinal agent. Up until about 80 years ago, cocaine was put in soft drinks.
This begs the question: what is worse, using cocaine one time, or getting drunk two or three times a week (as many American high school and college students do, and maybe some other tour players do)?
You're using the term 'begging the question' incorrectly. But your right, it does raise the question of which is worse. And seeing as cocaine is both illegal in her home country, in the UK, where Wimbledon is, and BANNED by the WTA, I must say that the cocaine is worst. If you find the rules irrational, then you either follow them, refuse to follow them and quit playing on the tour, or break them, and face the consequences when you're caught.
That said, I don't know if she used coke or not. But nothing so far has led me to believe that Hingis' word is more reliable than drug tests.