Why Is Homeland Security Buying 450 Million Rounds of Hollow Point Bullets?

The Department of Homeland Security (DHS) and Immigration and Customs Enforcement (ICE) has awarded defense contractor ATK with an Indefinite Delivery/Indefinite Quantity (IDIQ) agreement for .40 caliber hollow point ammunition. According to an official ATK press release, U.S. agents will receive a maximum of 450 million rounds over a five-year period.
The following is an excerpt from the press release:

ATK announced that it is being awarded an Indefinite Delivery/Indefinite Quantity (IDIQ) agreement from the Department of Homeland Security, U.S. Immigration and Customs Enforcement (DHS, ICE) for .40 caliber ammunition. This contract features a base of 12 months, includes four option years, and will have a maximum volume of 450 million rounds.
ATK was the incumbent and won the contract with its HST bullet, which has proven itself in the field. [...]
“We are proud to extend our track record as the prime supplier of .40 caliber duty ammunition for DHS, ICE,” said Ron Johnson, President of ATK’s Security and Sporting group.