Specifically, the act requires financial institutions to keep records of cash purchases of negotiable instruments, and file reports of cash purchases of these negotiable instruments of more than $10,000 (daily aggregate amount), and to report suspicious activity that might signify money laundering, tax evasion, or other criminal activities.
http://en.wikipedia.org/wiki/Bank_Secrecy_Act
___________________________________________________________________
According to the inflation calculator, $10,000 in 1970 is worth about $1600 today. So the government didn't have to do a thing to increase the scope of the law. Just let inflation do its magic.
Thanks, Drug Warriors.