There's not a whole lot about the FMCSA that's particular logical. But with hazardous materials it does follow a certain logic. The purpose of the HAZMAT regulations is to protect the public, the environment, and the driver, in that order, from unreasonable risk of harm when transporting hazardous materials. There are some materials which present such high unreasonable risk when transported in any quantity (Table 1 materials) and other materials which present little or no risk when in certain quantities (Table 2 materials) but begin to pose more of a risk as the quantity increases.
As the level of risk increases, so does the packaging requirements and the level of training and education needed to safely transport it.
A 900 pound pallet of batteries or a 750 pound drum of paint might cause a real mess in a spill or an accident, but it doesn't present an unreasonable risk to the public at large, the environment, or even the driver. But the Secretary of Transportation has determined that more than 1000 pounds of those materials begins to pose more of an unreasonable risk to either the public, the environment or the driver, and thus requires the proper education the training to transport them.