Students from advantaged backgrounds should treat “recommended” as “required”, but the college lists them as “recommended” because some students from disadvantaged backgrounds may not have them (or may not have even heard of them until it was too late to take them in time for college applications).
Only for some divisions or majors at some campuses:
http://admission.universityofcalifornia.edu/freshman/requirements/examination-requirement/SAT-subject-tests/
Probably depends on the high school. Back when I was in high school (a non-elite public school which sent about 1/3 of graduates to four year colleges, mostly state universities), taking them (then called Achievement tests) after completing the associated high school course was not difficult earn a high score with no additional preparation besides the associated high school course (e.g. score in the 700s on the physics test after regular high school physics, score of 800 on math level 2 after precalculus). But if the high school is of really poor quality, then it may require more preparation to fill in the gaps.