I think it is more of a testing issue. The part is tested for that spec of the electrical characteristics table at a supply voltage (Vs) of greater than 3.3V. I see no reason why you cannot use it below this voltage. The 0.75V spec mentioned later is the saturation voltage of Q2 in FIG 1.
I thought that the input offset voltage of the sense amp would suffer below Vs = 3.3, and this might explain why the part is only tested above 3.3V. However, looking at the spec of Vos vs negative supply voltage, the input offset voltage is still respectable, hence accuracy will not be compromised.
So, I think it is just a testing issue