This is the default behavior in Gtk and I like it a lot. I never ever want to move one page up/down and don’t think a lot of people do, especially in an era with scroll wheels or two fingers gestures.
It’s kind of a confusing behavior at first, because even if it’s consistent with several operating systems, when I click somewhere I aim at that place. Also it’s the behavior we see on many contacts applications on mobile phones, where the right side of the screen is a slider.
People still have Page up and Page down keys if they choose this option, and I feel it’s consistent with the fact that you can e.g. configure if we want to use a single or double click (and I feel it should have its place there).
Besides, my comment could be extended to sliders.
*** This bug has been marked as a duplicate of bug 379498 ***