I share your disappointment. The reason for not doing it on bitshares or others was that that would have required them to setup a server or bot on bitshares to handle the distribution. Pretty weak excuse if you ask me, since all the proceeds of the auction will go to a single party. The trick they seem to use is that block.one will have nothing to do with the launch of an eos blockchain, anybody will be able to launch the official eos chain.

 Maybe they just want to flash crash the eth-price to help launch eos.

Another thing that comes across as a little icky, is that not only does block one get 90% of the auction proceeds, but they get 10% of the network as well. If block.one employees are able to use the proceeds to get a bigger stake in eos, that does sound kinda bad.