This is just another instance of North Americans (I haven't really seen this in British English speakers) placing their commata not at the boundaries between clauses/phrases, but where they pause when they read the sentence out loud. You may argue with descriptivism -- that the grammatical rules have changed and this is the new normal -- but placing a comma like this has the probably unintentional effect that reading the sentence out loud now causes you to pause in yet a different place.
I also wouldn't say that German has "otherwise the same grammar as English". (Or in wrong German: "Ich auch würde nicht sagen dass Deutsch hat ansonsten das gleich Grammatik wie Englisch" - even if we're just talking about comma rules, the German version should have a comma before the "that/dass").
[1]: http://www.neue-rechtschreibung.net/2012/04/30/kommasetzung-...
German and English have similar grammar but they are very far from being the same. Particular here with commas. Clauses in German are almost always marked with commas. English uses the comma much more sparingly.
In English there's also a difference in comma usage with restrictive and nonrestrictive relative clauses (restrictive relative clauses, which indicate which specific entity is referred to as opposed to others, don't use commas, while nonrestrictive relative clauses, which merely add additional information, do), but I seem to remember that native German speakers will commonly write both with commas.
*The person, who was here yesterday, has come back.
Conversely, it's sometimes hard for me to remember to use that comma in German. I want to write something like *"sie sagt dass man hier kein Komma braucht".