Every country has their own specific reasons for accepting or not accepting refugees into their country. There should not be a set of standards laid out for every country. If they want to use potential terrorism as a reason for turning refugees away then that is their right. We cannot force a country to do anything.
Yes, the potential for terrorism should be a key concern when deciding on letting refugees enter the country. ISIS has explicitly stated that they intend to seed the flow of refugees with sleeper agents for years. Now there are terror attacks taking place with alarming frequency across Europe. Why would ISIS act differently if America opens the floodgates, like Europe did?
Terrorism must be a consideration for a nation before it decides to admit refugees from any country. However, the threat of terrorism should never be used to discriminate against refugees from Muslim nations. Instead, a strict vetting process should be implemented to screen every refugee before entering a country. That way no group is singled out and discriminated against.
It is impossible to isolate terrorism since it is a global problem impacting all countries. The decision of allowing Muslim refugees is a sensitive issue for Germany and all countries. There does need to be a screening process put in place to identify and research those people who are gaining access into a country.