I bet it has everything to do with DLink not being in tune with their customers and their expectations.
While I can see that not many SOHO setups would have an internal NTP server, I can't see why you would block the service coming from an internal location to perform the sync. Why on Earth would you want to go to the effort of not allowing this.
The only reason DLink have their own NTP server is because they were sued by NTP.org for abuse as it was not set up for companies to build into their products. This was a free service for users and if they choose to use ntp.org on their router, then fine, but not for companies like DLink to market their products with it.