Maybe because the homeless or impoverished get internet in libraries and nowhere else, but regardless of the cause, something should be done.
The last time I used a computer in a library, I reached down to grab the side of the chair to scoot it under the desk, and received a handful of ejaculate for my trouble. It was a nice library too.
One of my best friends works in a different, nicer library, and constantly complains about people jerking off in there. They catch them and kick them out, but it doesn't stop the problem any more than swatting mosquitoes stops that problem.
You may say mental illness because of your particular environment or upbringing, but I feel below a certain level of society it's the natural state of people-- if they're able to see a naked picture they pursue the pleasurable sensations it produces until that process completes itself. Altering the environment seems like a damn fine idea to me.