If a US company is involved, the US can access data no matter where data resides. Even in your European datacenter.
More appeals to come, and I kinda hope this does not stand in any general form. If it does, it deals another body-blow to US tech companies, already seem as entities of the US national defense establishment due to the various NSA revelations.
And, I wonder, what if a European country had operations in the US, would it be equally subject to a subpoena here in the states?