The regulation in question is part of a state law that was passed by the Washington state legislature in 2006. The State Human Rights Commission did not act unilaterally and simply create a new law on their own. 

This is the rule that I1515 was really trying to overturn. RCW 49.60 The right to be free from discrimination because of race, creed, color, national origin, sex, honorably discharged veteran or military status, sexual orientation, gender identity or the presence of any disability. This right shall include, but not be limited to the following;

(a) The right to obtain and hold employment;

(b) The right to the full enjoyment of any of the accommodations, advantages, facilities, or privileges of any place of public resort, accommodation, assemblage, or amusement;

Evangelicals and conservatives in this state wanted to get rid of the rights of transgender people to enjoy the freedom to be able to shop, hold a job, or use any public restroom facilities