ANGLE does not currently provide a point sprite implementation for level 9 hardware (found in tablets and phones). The implementation for better GPUs is provided using a geometry shader. When using point sprites on these devices, this geometry never makes it to the screen.
The tessellation could be done on the CPU by simply passing in the corners of a quad instead of the original point buffer.
Better yet, a unit quad could be passed in as the geometry, and the points buffer could be used with instancing to draw the same quad repeatedly with different positions/sizes. Since level 9_3 hardware supports instancing, we could reasonably support this on all current and new WinRT phones/tablets.