da* 16-50 f2.8 artificially restricted to f2.8?

I'm quite new to this lens, but it seems like pentax have restricted the aperture of this lens to f2.8, even with the lens wide open the aperture blades are slightly closed.
On closer inspection, the aperture leaver is stopped from opening fully by a little metal stopper that could be removed so is placed there by design so it makes me think its not just my copy?
Has anyone ever tried to removed the aperture restriction or is it just my copy of the lens that is doing this?