i bet money the 17% number is the number of businesses either directly or indirectly IMPACTED by the US presence, whether that impacton a per-business basis be the price of a weinerschnitzel or 50% of said business' revenues.
You may be right at that - as I said, my German isn't all that great. That said, I don't think anyone can deny that the US bases are an important component of the German domestic economy.