Any phone with a 480x320 screen and free on contract (at least in the US) is low end. The iPhone 4 would be midrange and the 4S would be high but not the highest end.
The 3GS is not free on contract in Germany. Fact is that there are cheaper phones and that may be of importance in markets where the average wage is lower than in the us. And since the article is about world-wide number of devices shipped, that makes a diffence.