Maybe they're not actually buying the ammo for themselves. Ever consider the alternative?
Like, perhaps this is a new version of "Iran/Contra" and DHS is just coordinating a lot of small purchases through other agencies to try and create a confusing paper trail for the purchase. Perhaps this ammo isn't for US agencies at all--perhaps the DHS is buying it for the Department of State or CIA or someone like that, to funnel to AlQaida "rebels" in Syria. We ARE, after all, supporting Al Qaida in Syria now, if you haven't heard. Even the Council on Foreign Relations is praising Al Qaida's efforts there...
http://www.cfr.org/syria/al-qaedas-specter-syria/p28782
I'm not sure which I find more scary and morally repulsive though--the idea that our own government is arming itself in EVERY department from the Post Office to the Department of Education, against the paranoid idea that the People will start a civil war, or the idea that our government is supplying arms and munitions to a group that is supposedly our most dangerous enemy on the planet.
Neither option would surprise me though.
This IS, after all, the same government that engineered the deaths of over 3000 Mexican civilians and 3 Federal LEOs under Fast and Furious, in an attempt to demonize the 2A...