As I understand it, a spouse's ticket would be absolutely free as well. Am I correct about that ?
Thanks so much !!!
I may be wrong, but I am almost positive this what I read when Delta made the Domestic Partner rule: Delta considers DP and spouse the same for travel privileges, but the IRS does not. The value of the ticket is considered income and therefore the tax must be paid.

Something similar was posted on Travel Net about DPs using ZED fares. Something along the lines of: While Delta does not make a distinction, not all airlines treat a DP the same as a spouse.

The short answer: Spouses are free domestically, and pay the same as employees for overseas.