They only take out any tax that wasn't paid. Since it is all automated, this is rarely the case.
If you ask for a copy of your earnings summary, you will see that you infact are taxed. It is a tiny amount.
When new laws are introduced, there are usually clauses to cover those who are following old statutes. Being foreigners, we don't get the usual benefits of fair treatment. So more than likely they will across the board change for all.
I am talking about the 2010/11 tax year.