< /paranoia > You forgot to close your tag.
Perhaps because the folks who stole the material, didn't know it couldn't be used for explosives.
Even if the stuff is 1% ammonium nitrate, that's still 75 lbs in the total that they took. That amount could be extracted and processed to result in ~8 bombs of 10 lbs each, the same as in each of the subways and busses in London. It's quite likely that the material is >1% ammonium nitrate, resulting in even greater yield.