To understand why this is the case, you need to read Josh's blog post.
Honestly, I think it's a silly idea and will do nothing to rebuild DreamHost's reputation (not that I think it needs rebuilding). As far as I can see, it just causes confusion, but hey, it's their company.
Thankfully, it does not affect sign-ups that use a promo-code, they continue to receive the old disk space / bandwidth quotas.
Save [color=#CC0000]$50[/color] on DreamHost plans using [color=#CC0000]PRICESLASH[/color] promo code (Click for DreamHost promo code details)