求解释~~Manhattan Chapter 6 的Problem set 第一题

小臣

原题为 Upon setting foot in the Gotnic cathedral, the spectacularly stained-glass windows amazed the camera-wielding tourists.

改错后为Upon entering the G cathedral, the camera-wielding tourists were amazed by the spectacular stained-glass windows.

Manhattan 的解释为：原句中upon为介词 upon setting foot  in the G cathedral 包含动名词。是谁或什么踏上了cathedral？逻辑上来讲，一定是tourists，然而，windows是主语，是setting的主语。

我不太理解 为什么要把setting foot in改为entering？
SC菜鸟一只~~求大牛指点！