Blogging on root blog

Hiya,

I've had multi-site working great for a while now. The user can create multiple sub-domains on multiple domains. However, my engineer claims that the functionality of multi-site only allows users to blog on sub-domains, but not domains.

For example,
sub1.domain1.com
sub2.domain2.com
sub3.domain3.com
all these sorts of domains are fine for blogging.

But when we try to land on:
domain1.com
domain2.com
domain3.com
we can't blog on these domain, and instead get automatically redirected to the multi-site primary install domain.

What are we missing?

Thanks,

Alberti