it's leased and as such, the lease gets the full benefits of "private property". The can ban firearms if they want (and they do)
A municipality acts in a proprietary capacity when it “acts as the proprietor of
a business enterprise for the private advantage of the [municipality]” and it may
“exercise its business powers in much the same way as a private individual or
corporation.” Hite v. Pub. Util. Dist. No. 2 of Grant County, 112 Wn.2d 456, 459,
772 P.2d 481 (1989); Branson v. Port of Seattle, 152 Wn.2d 862, 870, 101 P.3d 67
(2004). When acting in a proprietary capacity, a city may enter into any contract
“‘which is necessary to render the system efficient and beneficial to the public.’”
Hite, 112 Wn.2d at 460 (quoting Puget Sound Power & Light Co. v. Pub. Util.
Dist. No. 1, 17 Wn. App. 861, 864, 565 P.2d 1221 (1977)); see also Stover v.
Winston Bros. Co., 185 Wash. 416, 422, 55 P.2d 821 (1936).
Also...........Go read the Sequim case
http://forum.nwcdl.org/index.php?action=downloads;sa=view;id=24
Applying our reasoning in Cherry, it follows that a municipal property owner like a
private property owner may impose conditions related to firearms for the use of its
property in order to protect its property interests. For the same reason that a
municipal employer may enact policies regarding possession of firearms in the
workplace because a private employer may do so, a municipal property owner
should be allowed to impose conditions related to sales of firearms on its property if
a private property owner may impose them. The critical point is that the conditions
the city imposed related to a permit for private use of its property. They were not
laws or regulations of application to the general public.
Since Safeco is not open to "the general public" but rather only ticket holders, it would be considered a "private event", so again, they could restrict firearms if the desire.