I guess because it expects the output to be the same type as the input, and imnoise expects that if a double is being passed in it has to have a range of 0-1, so it clips anything less than 0 to 0. You would have to use randn() if you wanted negative values in a floating point image. Here is the corrected code:
clc; % Clear the command window.
close all; % Close all figures (except those of imtool.)
clear; % Erase all existing variables. Or clearvars if you want.
workspace; % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 20;
originalImage = zeros(255);
noisyImage = imnoise(originalImage, 'Gaussian', 0, 0.01);
% Display the image.
subplot(2, 2, 1);
imshow(noisyImage, []);
title('Noisy Image with imnoise()', 'FontSize', fontSize, 'Interpreter', 'None');
% Set up figure properties:
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);
% Get rid of tool bar and pulldown menus that are along top of figure.
set(gcf, 'Toolbar', 'none', 'Menu', 'none');
% Give a name to the title bar.
set(gcf, 'Name', 'Demo by ImageAnalyst', 'NumberTitle', 'Off')
% Let's compute and display the histogram.
subplot(2, 2, 2);
histogram(noisyImage);
grid on;
title('Histogram of noisy image from imnoise()', 'FontSize', fontSize, 'Interpreter', 'None');
xlabel('Gray Level', 'FontSize', fontSize);
ylabel('Pixel Count', 'FontSize', fontSize);
noisyImage = originalImage + 0.01 * randn(size(originalImage));
% Display the image.
subplot(2, 2, 3);
imshow(noisyImage, []);
title('Noisy Image with randn()', 'FontSize', fontSize, 'Interpreter', 'None');
% Let's compute and display the histogram.
subplot(2, 2, 4);
histogram(noisyImage);
grid on;
title('Histogram of noisy image with randn()', 'FontSize', fontSize, 'Interpreter', 'None');
xlabel('Gray Level', 'FontSize', fontSize);
ylabel('Pixel Count', 'FontSize', fontSize);
