Zero Frequency Basis Equation for Discrete Sine Transform
I think I got it: 

While in the DCT there are definitions of the DCT that make cos(0) when u and v are zero, this does not apply to DST (see: https://en.wikipedia.org/wiki/Discrete_sine_transform). 

In the DST definition, there is always a +1 or a +1/2 adding to the frequency. 

This explains why it is not zero and also the graph above. It varies 
(for u=0 and v=0) because the content of the sine is not zero.