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Not sure where the option is on the iPad version, but this is a preference setting that you can change on the Mac version--I suspect the iPad has a similar setting. On the Mac, the checkbox is called "When completing the last item mark the project or group complete".

The reason that it is configurable is that it is most likely not possible to plan ALL actions at the time you create the project--in fact, I think a lot of GTD'ers only add the next action, or the next few. That means that just because you mark the last task in the list complete, doesn't necessarily mean the project is complete.