Eric Holder, right, announced major changes to drug sentencing practices on Monday. AP Congress has yet to reform America's draconian mandatory minimum sentences, but Attorney General Eric Holder was able to step in and make some pretty big changes of his own on Monday.

Mandatory minimum sentencing guidelines force judges to hand down harsh sentences based on the amount of drugs people sell — regardless of mitigating factors such as drug addiction. These 1980s-era laws have been criticized for being both costly and cruel.

In a moving speech on Monday, Holder announced changes to the way that the Justice Department enforces mandatory minimums. He can make these sweeping changes without Congressional approval because he's not changing the laws themselves.

Under Holder's new guidelines, federal prosecutors will not charge defendants with dealing a specific amount of drugs if those defendants have committed "low-level, nonviolent drug offenses" and aren't a part of large organizations, gangs, or drug cartels.

If prosecutors don't charge a defendant with having a specific amount of drugs, mandatory minimum sentences can't be triggered.

"Prosecutors have a great deal of discretion with respect to the kinds of crimes that they charge," Mary Price, vice president and general counsel for Families Against Mandatory Minimums, told Business Insider.

The "prosecutorial discretion" Holder exercised falls pretty squarely within his authority to decide what kinds of crimes federal prosecutors charge, law professor Kevin Brown told us.

"As the chief federal prosecutor, Holder clearly has the authority to exercise prosecutorial discretion for federal crimes in this way," Brown said. "The only unusual aspect here is his public announcement — most prosecutors do not declare their prosecution policies publicly."

In addition to declaring his own policy, Holder announced that he would work with Congress to pass legislation that would reform mandatory minimum laws themselves. It's still important for Congress to step in since a future attorney general could easily change Holder's policy, Price pointed out.

"It's an important gesture because it means the Department of Justice is taking a first step," Price said. "It's not a lasting gesture because departments will change, and what we ought to do is change these laws."