> If I was her, I would fight for the email address "hillary@clinton.com."

How does one "fight for an email address"?

Once you own a domain, you own it. It doesn't matter that it just so happens to be someone else's last name.

She would have had to pay most likely a large sum to the investment firm that already owns clinton.com... and perhaps they aren't interested in selling, or they value the domain too high.