For starters, Germany declared war on Russia five days before Austria-Hungary did. But even if it were otherwise, they still had a choice whether to honor their obligations or not.

Furthermore, there's substantial evidence that Germany - or rather, some German factions - was very specifically pressuring Austria into a war, precisely so that it could then be "joined":

https://en.wikipedia.org/wiki/July_Crisis#The_German_%22Blan...