Can you adjust a 9 speed rear derailleur to work properly on a 8 speed casette? If so then why do they sell 8 speed and 9 speed derailleurs?

Yes.
Marketing.

Thanks dorin!

There's no fundamental difference among 7 to 10 speed derailleurs other than the decal that tells you how many speeds its for. As dorin said... its marketing. The real derailleur differences come in the rated cog sizes and capacity figures, but even those are usually conservative, again for marketting reasons (nobody would buy an XTR middle cage derailleur if they knew the ultegra middle cage model would work as well for them and weigh less at the same price).

I always thought the XTR line was just some kind of utopic fantasm from the guys at shimano, you know kind of like Atlantis...

I remember when it was introduced and to buy the group at a bike store was $1200, yet KHS had a model that year (1992) that had the group, a good steel frame, Flite Ti saddle, wheels, rigid fork, etc... $1400. Really put things into perspective as to how much things get marked up at the retail store.