Actually, you can't delete them as you don't have adequate permissions. Logs are an important part of our system management and audit tools, so DreamHost "owns" them and we delete/rotate them in accordance with our needs.
You may have deleted the domain, but we still may need the logs. This is normal on a shared server. I'm not sure why you are concerned about it, as it can't be a storage quota issue given your massive amount of allowed storage, and the fact that the logs we keep are not counted against your quota anyway (if you decide to enable more than three days worth of logs, the extra days do count, but as I said, it's really a moot point given your quotas).
--DreamHost Tech Support