According to this it's public:
Safeco Field, opened in July 1999, is owned by King County citizens and is overseen in trust by a municipal corporation -- the Washington State Major League Baseball Stadium Public Facilities District, commonly called the PFD.
More than 30 million people have attended an event at Safeco Field making it one of the most popular visitor attractions in Washington. More than 400 events -- from high school graduations to corporate meetings -- are staged at Safeco Field each year in addition to the 81 home games played the Seattle Mariners.
Safeco Field is ranked as one of the best stadiums in Major League Baseball.
The PFD leases Safeco Field to the Seattle Mariners which manage operations and maintenance for all events in the facility.