It's one thing to make sweeping statements about welfare of others and quite another to realistically look at what that means. I am nitpicking here but let's go over your list... I am going to break it down to how it really is in NYC (I presume LA is not that dissimilar):
burger king workers - unhappiest people you'll see anywhere... excepting teens and other temporary situations, completely unemployable outside the fast food industry
street vendors - mostly cash income, probably make more than reported $15/hour, probably have other jobs to support them through the cold winter months
bus workers - unionized, make up to $70k/year
broadway ushers - mostly volunteers or actors who only work until they "make it" and only to network with other theater people
busboys - mostly illegal mexicans with little other choice, excepting some high end restaurants where they make more than $15/hour and do it as an apprenticeship to better paying jobs
I think most people who say that $15/hour in a major city like New York or LA is pretty good would not themselves concede to subsist on that little while working that hard... and I know what I am talking about, having done some of those hard-labor jobs when I first immigrated into the US. It only works in temporary situations such as students or apprenticeships, or as a means to support higher artistic aspirations.
I would bet all of my money that not a single person on here (including those 6 who upvoted your comment) would choose a career of a $15/hour laborer for themselves, and I think it's a little patronizing to say that it's ok for others.