lens - How do zoom lenses restrict their widest aperture at the telephoto end?

Does the aperture ring lock the apertures beyond, say 5.6 at length lens's telephoto end? Does the lens introduce an obstacle to the aperture ring so the lens can't be opened anymore beyond the aperture at the telephoto end?

And why do lenses behave like this, anyway? Why don't they have constant apertures throughout their focal range?