Well, given the alternative and some modest regard for basic human rights (such as, e.g., stated in the German constitution [1]), they are legally bound to do exactly that.
Especially since the reports of spying activities from within the american embassy in berlin. In fact, it would have been necessary at this point already to investigate the embassy itself, and to ban the current american embassador if anything were found.
[1] http://dejure.org/gesetze/GG/10.html