why limit the num of numCategories in AbstractOnlineLogisticRegression

[myn]

I was read the AbstractOnlineLogisticRegression(SGD) sorce code  ,but
it limit the number of numCategories to 2,why? why not 3 or more?

@Override
  public double classifyScalar(Vector instance) {
    Preconditions.checkArgument(numCategories() == 2, "Can only call 
classifyScalar with two categories");

    // apply pending regularization to whichever coefficients matter
    regularize(instance);

    // result is a vector with one element so we can just use dot product
    return link(classifyScalarNoLink(instance));
  }