Ahhh, still a referral though. I just generalized with cookies because a large percentage don't even know they're sending someone else money, or in many cases, a friend sends a link, they click a link on a site they don't mind supporting, etc...
In your case though, even if you had unknowingly clicked a referral link at some point, at least you overwrote it by entering your friend's email, making sure he gets the credit. He at least bought you lunch with part of it, right? 
I'm mainly just wondering what percentage of their sign-ups don't cost them $97.
It's just like how some people think the top section of search results are actual search results, rather than paid ads that put money in an affiliate's pocket.
I think a lot of people see dreamhost.com in the link and don't really pay attention to the stuff at the end... or they get redirected and don't notice.
I liked it better when you couldn't make promo code links. I think it keeps things more legit when the customer has to enter what he wants, whether it's a promo code or a friend's email address.
You could actually get screwed by a promo code link. Let's say you tell a friend to enter "USEMYPROMOCODEORDIE" when he signs up. He sees the discount automatically applied and figures he doesn't have to, not realizing it was because he got cookied earlier while reading up on DH.
I doubt it's a common problem, but without the links/cookies, it can't happen at all.