How can we understand what caused Henry George to "lay down the law," so to speak, by establishing the September 1890 Platform of the Single Tax League of the United States (Platform), and then requiring anyone claiming to be "Georgist" to acknowledge the Platform as the most accurate representation of his intentions?

Here is a Suggested Reading List, which I think anyone trying to understand the national Platform will need to reference:

(1) Progress and Poverty (1879) by Henry George: essential for understanding what started the movement, and for providing essential definitions of terms necessary for establishing what George called the "science of political economy."

(2) The Standard (1887-1892) by Chief Editor Henry George: after the election of 1886 exposed many misconceptions about his "single tax," George began editing this weekly newspaper to address them. Although there's almost too much to read here (the DVD's contain all 296 issues), it's only way to see what led George to establish the 1890 Platform, and then to require allegiance to it.

(3) A Just and Practicable Income Tax (1893) by Hon. Thomas G. Shearman before Ways and Means Subcommittee on Internal Revenue: This 21-page proposal by Henry George's lawyer, Thomas Shearman, was rejected by the Subcommittee, but contains a description of kind of income tax which the 1890 Platform engendered. The income tax did not included wages in the tax base, and minimized income tax evasion by targeting the payers of unearned income wherever possible, not the income receivers.

(4) Nature's Gifts: The Australian Lectures of Henry George on the Ownership of Land & Other Natural Resources (2014) by John Pullen: essential to understand how the 3-month 1890 Australian tour caused George to more accurately define his "single tax." After returning to New York he established the September 1890 Platform, and a few months after that had a minor stroke.

(5) Henry George and the Crisis of Inequality (2015) by Edward T. O'Donnell: essential to understanding Henry George's mindset and reactions after being heavily criticized when he ran for mayor of New York City.

(6) Natural Taxation: An Inquiry into the Practicability, Justice and Effects of a Scientific and Natural Method of Taxation (1898) by Thomas Shearman: essential to understanding how the Georgist movement handled the defeat in Pollock v. Farmers' Loan (1895), which effectively ended the national movement, and caused it to revert back to using state and local property tax laws. George died in 1897 and Shearman in 1900, so neither was alive to revive the national appeal when the 16th Amendment made their national "single tax" possible again, and no Georgist leader to date has corrected the situation.