That's actually more a Tribal Politics charged move then you may think.

Its worth noting that the Duwamish, under the direction of Chief Seathl, where the only tribe to come to the aid of the city that bore his name during the Battle of Seattle (1856).

The Duwamish divided into two different groups back in the late 1850's with the significant majority banding together with the Buklshuhls and many other tribes on the reservation to become what we call the Muckleshoot; that group inherited the rights as signers of The Treaty of Point Elliott. The other smaller group eventually banded together (or stayed together, the records of their leadership during that time are pretty much non existent) today call themselves the Duwamish, but are not a federally recognized, as the Muckleshoot are viewed as the direct political successors of the Duwamish (1855) that signed the Treaty.