Once you look at voltage sag under the high current, and throw in some voltage loss in the circuit (especially at high amperage) you are best case getting limited regulated runtime. Worst case with some cells you aren't able to get the full 3A current unless the circuit boosts. I'm sure there would be a lot of unhappy people here over not pushing the spec'd current.
Of course they could use a boost or buck/boost circuit. Building the light around a buck boost would not only be less efficient, it generally comes at the cost of not supporting 2xLi-ion voltages. Throw in the fact that boosting takes you to or above the limit for some lower capacity 18650's sustained max current and a single 18650 just doesn't seem like a wise design choice for this light.