You're assuming that vignetting is an exception in the camera world. The pics you posted aren't anything strange, vignetting just happens and it doesnt depend only on sensor size, lens and aperture.
one picture is better than 2000 words (whether they are my words or yours).
So enjoy these pics taken with a canon 5D.
The first one with a 24-105 f4.0L IS, at 24mm, f4.0:
http://www.lucianomorpurgo.com/temp/IMG_8229.jpg
second one, canon 5D, EF70-200 f2.8L IS, at 200mm,
f7.1 :
http://www.lucianomorpurgo.com/temp/IMG_8270.jpg
(just guess how it would be at max aperture)
Vignetting depends on a lot of things - just to make an example I rarely get vignetting at f7.1 with any lens but this example is an exception - depending on some particular light condition.
Luc