This, some argue, gave employers the ability to require more from Bracero workers based on a threat or promise they would be sent back to Mexico.
No one argues that employers lost the ability to require anything of Bracero workers after their threat. 
mnli hypothesis: No one argues that employers lost the ability to require anything of Bracero workers after their threat.  premise: This, some argue, gave employers the ability to require more from Bracero workers based on a threat or promise they would be sent back to Mexico.