Not an expert on the Dewalt tools here, but with the Lithium cells used in this pack it is important not to drain them below a certain voltage, otherwise they get permanent damaged.
So manufacturers have to put a protection either direct to the battery or to to the tool the battery is used in.
When using NiCad it doesn't really matter, NiMh also doesn't like to get deep discharged, but they loose power at the end of charge so people usually recharge early enough. There are some protected NiMh packs, though.