It's true that allowing a spouse as an extra driver with no extra fee, varies by company and by location (ie country).
Other companies generally charge an extra driver fee for a spouse, with the following exceptions:
No fees are charged for a spouse who is an authorized extra driver in California,Illinois, Missouri, Nevada, Oregon, and Wisconsin.
Emerald Club Members in the United States and Canada are allowed to have thier spouse, common law spouse, or domestic partner who has the same address following added asw an Additional driver without a fee.
All of my research indicates that every company outside of North America charges an extra driver fee for the renter's spouse. I've checked with all the major brands, but there may be small, independent car rental agencies in the country that you're looking to rent in that may waive that additional driver fee for spouse; you would have to check the policy for each individual company/location.
*Note: Car rental companies can modify policy at any time; I make every attempt to maintain up-to-date info, but always check with an individual company to verify any policy. See: Disclaimer
Pages from this site, relating to this topic:
Car Rental Policies Index
Additional Drivers Overview
Company Specific Additional Driver Policy