Can't crawl Amazon subcategory data

I am writing a simple API that takes a set of keywords as input and returns the Amazon categories matching the keywords

As following picture: ![1]: http://ift.tt/1Ov2mA5

After input the keywords, I want to get the categories text in the left side.

So I wrote a simple Python code (urlopen) to fetch the page. But the program only downloaded incomplete content, I rendered it on browser as following

![2]: http://ift.tt/1xZYpJk

As you can see, the left side area does not have subcategories, what should I do to get the complete data? I tried Amazon Product Advertising API, but it does not have detailed categories information. And I also did some research, I think I need to trigger some Ajax event to get the data. But I don't know where to start.