This is a horrific and tragic attack. At the same time, we need to realize that terrorists intend to have a psychological impact beyond the actual physical harm that they cause. It's important to understand their methods, motives, and mindsets just as a doctor strives to understand a disease.
This is early speculation, but I'm guessing that these particular terrorists have a bigger axe to grind against the Indian government than they did against the Americans and British that they were ostensibly targeting. I think that their goal is to scare Americans and British and keep them from traveling to India, thus damaging the Indian economy. According to the article, eyewitnesses said that the gunmen asked who had US and UK passports. Did they really expect everyone with those passports to raise their hands? Probably not, so I'm guessing that they wanted terrified survivors talking to reporters.