They were once America's cruelest, richest slave traders. Why does no one know their names?

[mahatmakanejeeves]

13. "The law prohibiting slave trade had gone into effect in Lincoln's presidency,..."

If you're referring to the prohibition of the importation of slaves, it was someone else on Mt. Rushmore who did that.

Act Prohibiting Importation of Slaves

Long title: An Act to prohibit the importation of slaves into any port or place within the jurisdiction of the United States, from and after the first day of January, in the year of our Lord, One Thousand Eight Hundred and Eight

Enacted by: the 9th United States Congress
Effective: March 3, 1808
Citations: Public law 9-22
Statutes at Large: 2 Stat. 426
Legislative history: Signed into law by President Thomas Jefferson on March 2, 1807

The Act Prohibiting Importation of Slaves of 1807 (2 Stat. 426, enacted March 2, 1807) is a United States federal law that stated that no new slaves were permitted to be imported into the United States. It took effect in 1808, the earliest date permitted by the United States Constitution.

This legislation was promoted by President Thomas Jefferson, who called for its enactment in his 1806 State of the Union Address. He had promoted the idea since the 1770s. It reflected the force of the general trend toward abolishing the international slave trade which Virginia, followed by all the other states, had prohibited or restricted since then. South Carolina, however, had reopened its trade. Congress first regulated against the trade in the Slave Trade Act of 1794. The 1794 Act ended the legality of American ships participating in the trade. The 1807 law did not change that—it made all importation from abroad a crime. The domestic slave trade within the U.S. was unaffected by the 1807 law. The United Kingdom, the major power involved in the Atlantic slave trade, had passed the comparable Abolition of the Slave Trade Act, on February 23, 1807 (achieving royal assent on March 25, 1807).
....

Background

Article 1 Section 9 of the United States Constitution protected the slave trade for twenty years. Article 5 said this clause could not be affected by constitutional amendment. Only starting January 1, 1808, could there be a federal law to entirely abolish the international slave trade, although individual states could and did ban it at any time.

The Migration or Importation of such Persons as any of the States now existing shall think proper to admit, shall not be prohibited by the Congress prior to the Year one thousand eight hundred and eight, but a tax or duty may be imposed on such Importation, not exceeding ten dollars for each Person.

In the words of Sarah Palin, slavery was one of our God-given rights.