There is no economic principle or anything of the sort preventing Germany from signing a trade agreement with the United States, only an unenforceable compact. Sure they have an agreement to have a common market, but these are sovereign nations. Agreements only last as long as they want them to. Germany is no more able to make a side deal with the US in 2017 than Italy was free to make a side deal with the UK in 1915. The difference is, if Italy had violated her treaties that required her to help Germany, there was a whole country full of angry Germans with the finest military in Europe, just over the mountains. But they still did it, and arguably, got away with it to some extent. Nowadays, if Germany does make such a deal, who is going to enforce their adherence to the common market? Trump, not being a pedantic fuckwit probably grasps this point, while Merkel tried to hide behind treaty nonsense and play the politician's favorite game of pretending her hands are tied.