I'm pretty sure I could continuously stream 3Mbit/s without using much CPU - that kind of load hardly makes a blip on my home server. The reality is you'll always be sharing the pipe with other people so it would be hard to ever reach that amount except for bursts.
I think the real trick is that 1TB/month means - if you can use that much its included, but hah just try it! Kinda like the free 48 oz steak - if you eat it all in one go!
But the reality is even at 500GB or 200GB or heck even 120GB per month it was a 'king great deal!
As for disk space - well lets face it, disk space retail is significantly less than $1G per GB, probably in the vast quantities DreamHost use its more like $0.50 per GB.
As the DreamHost blog mentions, their biggest expense is power and machines consume a certain amount of power regardless of whether their bandwidth pipes or disks are empty. So its in DH's best interests to ensure ever shared host machine is as jammed packed with users as possible. Every minute they are running less than 100% CPU is another minute a less than the ideal customer to power consumption per server ratio.