Doesn't hurt that so many of them are dual citizens, like Green. But yeah, the Germans certainly don't mind bringing them in.
There are way more than dual citizen Americans playing in Germany. There is actually a fairly long history of our players going there, at least by US standards. Claudio Reyna, Eric Wynalda, Steve Cherundolo, Frankie Hejduk, Greg Berhalter, Tony Sanneh and others spent time in Germany. Cherundolo was a one club man for Hannover 96 and he still coaches there. He recently retired and the club even held a testimonial for him, which is one of the highest honors a club could bestow on a player. Bayer Leverkusen was the first club to sign Landon Donovan and owned him for years. After Landycakes ran roughshod on Germany in the World Cup in 2002, I believed Bayer Leverkusen tried to recall him from his loan to MLS. I was at that game in 2002 and one the things that few American fans realize is that there was a ton of Bundesliga experience on the US team. So even though Germany was the second best team in that World Cup, we matched up to them decently because there was a level of familiarity (hmmm maybe that's another reason why having more players in Europe is a good thing).
My personal opinion is that England isn't the greatest place in Europe for American players. For one, it's a meat grinder. If you're playing for a good team, you're going deep in two Cup competitions, League and Europe. Ever notice how tired the English players look in the World Cup?
Secondly, when a player has a bad game in England, particularly a non-English one, there isn't much forgiveness. In the Bundesliga, Serie A and La Liga they seem to have way more patience with younger players. That was one of the reasons Emerson Hyndman left Fulham. There simply is no patience in England. Look at Jurgen Klopp in Liverpool. Just this week he said that he can't do what he did in developing players at Liverpool like he did at Borussia Dortmund because the club wants results now. That's the way it is at nearly every club over there. They want results NOW. That's fine and all, but in terms of development our players should probably steer clear until they are closer to a finished product.