The reason is pretty simple. The German craftsmanship is underpaid and pretty much everyone moonlights a little. In addition the German financial authority is worse than the NSA. No transactions no questions.
Nefarious types can leave their phone at home or ditch it before crossing a border. And just buy a new phone after crossing a border to connect to their existing phone plan account. It's easy for these persons to bypass security checkpoints and still connect with their regular contacts abroad. Ultimately it's just another useless "security" measure.