The point of this chapter in Zinn’s book is that the United States government has consistently abused the Native Americans. The title refers to a phrase used by Andrew Jackson in giving instructions to an envoy to the Choctaws and the Creeks. The envoy was to tell them that they should accept Indian Removal and move out beyond the limits of the United States. If they did so, they would own the land in that area “as long as grass grows or water runs.”
Of course, the US government did not keep that promise. Native Americans were later pushed off lands west of the Mississippi as soon as white settlers wanted that land. Therefore, Zinn uses this phrase as a title to emphasize how Native Americans have been (in his mind) mistreated and lied to by the US government.