If you define your own windows it is easy to mix up the time characteristic with the wrong set of predefined WindowAssigners. You cannot use processing time with a TumblingTimeWindows window assigner, for example.
Neither from the name of TumblingTimeWindows nor from the JavaDocs it is clearly obvious that this WindowAssigner can only be used with event time. I think it would be better to rename the event time window assigner to something like TumblingEventTimeWindows. Additionally, we could extend the JavaDocs a bit, since not everyone knows that "based on the timestamps" means based on event time.