It is not so much about trust re: BSI and T-Com as it is about control. The German government can legislate about what German based entities can and cannot do with German citizens' data - and can back this up with some serious legal punishments including massive fines and imprisonment for transgressors.
However, The German government has basically no say in what foreign companies do in foreign countries - all they could do is mandate a set of criteria before they are allowed to do business in Germany (in the way Saudi and the UAE are doing with RIM).
It is likely that the German authorities don't want to get into a pissing contest with Apple, RIM and similar as it would prove massively difficult to enforce and would be very unpopular with the German voters if Apple / RIM called their bluff and millions of German citizens lost access to their iPhone or Blackberry.
Yes, encryption etc would help to a point, but it still wouldn't allow Germany to control German citizens' data.