"If you don't want them viewing the internet then tell them to stay away from the computers."
You must not know much about computer display technology? These monitors are fascinating gizmos that display images visible to a fairly wide region. While it is quite easy to completely ignore a random book on a shelf, it is a bit more difficult to ignore a publicly facing computer screen - and by ignore I don't mean just "pay it no attention" but "keep from seeing the images being produced on the screen." If I go into a library, I can easily ignore books that don't interest me, but computer monitors draw a lot more attention, whether or not one wants to pay attention. Add children into the mix and it becomes much easier: I can tell my kids they aren't allowed to read the hot and heavy romance novels, and I can easily ensure they do not do so. But how easy is it to ensure they do not look at the publicly facing computer monitor in the middle of the library?