Does work experience in non-related field help in grad school admissions? USA: Does work experience help for graduate admissions non-phd in STEM fields?

Someone told me yes, because if I already have a job, the grad school can be assured that I'll have a job after graduation and I won't hurt their statistics on job placement among alumni. 

I disagree because I think that if the program is rigorous, all students who graduate should because of the greatness of the program be able to find jobs.

ONly if the program is insecure about their program's usefulness and quality should the program decide to admit students on whether they've already got a job. cough cough lots of MBA programs...  I believe only diploma mills would care about whether the applicant already has a job to decide whether they should get admitted.
It might have been useful to state the country you are inquiring about as this may be different all over the world.

However, in my experience, the people who oversee the admission process are not at all related to the people who create the statistics. They admit you because they believe you could handle the program - not because of some job prospects. Relevant job experience could be one factor which makes it more likely that you succeed (or not).

Indeed, most professors I know see their programms not as a way to obtain a (non-academic) job, but rather as a way to spread deep academic knowledge.  (I don't disagree with this view.) I don't see how a good academic program would by itself mean a job guarantee.

Edit: I just saw that you write "non-related" field in the title. Again, there is no yes/no answer to this question. Some professors might see this experience as positive since it might show that you have certain skills, others may see that you are not loyal enough to their field..