They're not - it was just the first thing that jumped to mind when I saw those images. Then I went back later and noticed that the website had them labeled at the bottom as "Asperatus". The images were impressive enough that I never even noticed there was type at the bottom and had never heard of Asperatus clouds before.
It wasn't until I started playing with Terragen a few years ago that I was even aware of so many varieties of clouds. I always thought they were limited to just the obvious ones - Stratus, Cumulus, etc.
But its like anything else on the internet: no matter how obscure your interest is, you're sure to find someone with a website devoted to it. (For me its pylons. Not the orange traffic cone-variety, I mean the high tension power-line type. And yes, I've found websites about them!)