So that the US can do the "dirty work".

We give the data to the US and, of course, the US only uses the data to fight terrorism.

I think our politicians knew that the US was doing more with the data than they said. But since this kind of data usage is not really aceptable in large parts of the EU we simply gave the data to the US to get the job done.

Then the politicians could say "we are not using your data and the US is only using it to fight terrorism" but in fact there was much more going on and everybody knew that.