According to "What Happened on the Trail of Tears", why did many Cherokee people believe the Treaty of New Echota was illegal?

Most Cherokees opposed removal. Yet a minority felt that it was futile to continue to fight. They believed that they might survive as a people only if they signed a treaty with the United States. In December 1835, the U. S. sought out this minority to effect a treaty at New Echota, Georgia. Only 300 to 500 Cherokees were there; none were elected officials of the Cherokee Nation. Twenty signed the treaty, ceding all Cherokee territory east of the Mississippi to the U. S., in exchange for $5 million and new homelands in Indian Territory. More than 15,000 Cherokees protested the illegal treaty. Yet, on May 23, 1836, the Treaty of New Echota was ratified by the U. S. Senate - by just one vote.