It's county by county in CA. 2/3rds of the counties I work in, ER docs do not write (or release) 5150s. This came up at CalACEP once and people voted against trying to lobby to make it statewide legal for ER docs to do so.
I would personally like the ability to rescind 5150s that I think are clearly nonsense, however I would like some legal protections if I do so and it's not entirely clear that
Section 856 of the CA Code confers that. I think most people just don't want to deal with it and would rather turf it to mental health social workers and psychiatrists even if this means 100+ hour ER stays and demented/drunk patients being brought in on inappropriate holds.