That's a good point. In fact, there are a number of countries that, like the United Kingdom, have put a burden on the advisers, so that you can prosecute not just the people who are evading taxes but also the people who have advised them to do that. Also, what some countries have done is that if you have a transaction with a tax haven, they will not let you deduct certain expenses--that's quite a powerful tool--or they will reverse the burden of proof. In other words, it's the taxpayer who has to prove why he's using that haven, not the tax authority having to prove why the taxpayer wanted to use it.
On February 1st, 2011. See this statement in context.