"Arrivals from abroad could make the problem worse."
Of course. But I'm sure the astute German officials know it isn't just about having more people on hand. There's a big difference between German citizens coming home and foreign visitors, who often plan to tour around from place to place and might innocently spread the virus.
I'm also betting that they are focused not only on infection rates in different countries but on country-by-country characteristics. If the German authorities are cued in at all on the American media, they've learned that at home, Americans (and maybe other foreigners) have shown a significant pattern of disregard for their own country's covid-rules. Why admit them to Germany, if they are unlikely to follow Germany's guidelines?
OTOH it's not clear to me what the purpose of the ban on "high-risk" foreigners might be. It's not like the whole country would be knocking on Germany's door. Isn't the test, administered free of charge at the airport, supposed to send infected foreigners packing? Is the test they use unreliable?
As this virus period drones on, restlessness among Germans is budding, and like US authorities, the Germans don't seem to be breaking up the mask-averse crowds there. I think sometime soon they will be dealing with the same sort of resistance that is now pretty common in the USA:
https://www.dw.com/en/germany-protests-coronavirus/a-54456654