I bought some software from one of these sites several years ago and it worked out well.
I tried to research whether it was legal and what I found out at that time was that the software manufacturer was aware of these sites and did not like them but at that time it was legal.
A lot of this stuff is OEM which means it is supposed to be tied to a new computer and you are not able to transfer it to another computer. How it works is a computer builder orders a bunch of OEM software which is usually 1/2 price and then marks it up and dumps it on the web. Don't know how you get $50 Cs4 extended.
The software I bought i got a low price and I was able to register it and use it for an upgrade later on. It was a Microsoft product and if they said it was legit then who am I to argue?
Don't know how it fits in to the piracy argument. I used a legit credit card on a site that looked legit.