The NFL changed the rules after the Russell debacle that saw the Raiders grossly overpay their rookie quarterback. Russell infamously held out after Oakland drafted him in 2007. He skipped their entire training camp as he held them to ransom before they finally broke (via Bleacher Report).
Finally, they gave him a $68 million contract with roughly half of it guaranteed. Eventually, the NFL and NFLPA agreed to create a new rookie salary cap that they nicknamed the ‘JaMarcus Russell Rule.’ This stopped franchises from throwing cash at unproven players like the former LSU star.