On December 16, 2002, President George W Bush signed National Security Policy Directive (NSPD) 22, and it was entitled "Combating Trafficking in Persons"
As a result of that, on January 30, 2004, the Deputy Secretary of Defense forbid involvement with trafficked persons by US Troops as well as Government Civilians and Contractors stationed with troops, as they were under the Uniform Code of Military Justice (UCMJ) while so stationed.
This was because of the Military Extraterritorial Judicial Act (MEJA) of 2000, which states that all Government Civilians and Contractors who are stationed with troops in sites worldwide are under the rules of the UCMJ .. AND ... that anyone who does something which would be illegal in the US will treated the same as if they did it in the US, no matter where in the world they are.
On September 16 2004, the Secretary of Defense required training for all US Troops on Trafficking in Persons (TIP). It was one of those training courses that has been used as a template for what is trained now. No one knows the author, as it (the actual training booklet) is long gone. Every training course since has been worded pretty much the same, including the one I just took about 2 weeks ago.
In 2006, the Federal Acquisition Regulations Subpart 22.17, Combating TIP rule was established. The FAR are the rules ALL companies which contract with the US Government, within the US, must follow.
Finally, in 2011, the Defense Federal Acquisition Regulation's (DFAR) TIP rule was instituted. The DFAR is the regulation under which every company that wants to contract with the US Government, Department of Defense or the Intelligence Community, is part of and will follow, in order to compete on contracts. It was then that all Defense-related companies, were required to have a TIP statement. The reason the DFAR TIP rule was established, was because companies which contract with the US Government for contracts outside the US (i.e. private security firms contracted to provide services in the Middle East) were not necessarily required to abide by the rules established for companies within the US. By putting the TIP requirement into the DFAR, the US Government extended the TIP requirements to basically all companies contracting with the US Government, from anywhere, for anything.
The TIP statements of all companies are essentially the same these days, as the contracts which are written are all based on the DFAR and it states which must be done. If you have a company, and wish to bid on a contract with the US Government to make anything Defense-related, your company will have a TIP statement in it's company manual.
I would also point out that the same is true elsewhere. The UK Modern Slavery Act 2015 (the UK Modern Slavery Act), the Australian Modern Slavery Act 2018 (the Australian Modern Slavery Act) are pretty much exactly the same and have the same requirements that anyone contracting with them have statements in their contracts.
Once the rules forced companies to have TIP verbiage in their contracts, it became apparent that if their personnel were found to be in violation of the regulations they would be in jeopardy of losing those same, lucrative, contracts, so all of the companies started putting TIP language in their corporate policies, so that if/when their employees were found in violation, they could righteously tell their contracting officers that those employee's have been terminated as violating our own corporate rules, see? It's stated right here.