Bottom line is that DH has to pay some amount out of pocket to register a domain name. There's no "free trial" for a domain registration, and that's not just a DreamHost gotcha.
I never tried the Free Trial, so I don't know if there's some alert about paying for a domain name registration. I hope there is. In the end, you get to keep that domain name that you wanted anyhow and can later transfer it to another web host if you don't stick with DreamHost.
If you do stick with DreamHost, then you get a $9.95 credit per year to cover the expense of a domain registration.