I generally don't tip in the USA, but do tip consistently and generously when traveling in countries where that is expected. That's based on my perceptions of cultural norms.
After my second or third visit in Latin America, spending time observing in some less-touristy locales, it suddenly dawned on me that tipping for services provided was a form of income distribution. If you are wealthy (and in most countries any visiting Norte Americano is almost assuredly wealthy by local standards) it is expected that you will receive services from other members and will tip for those services.
Shining shoes, for example. My perception is that wearing leather shoes, marks a person as wealthy. And culturally, if a person can afford to wear leather shoes, as a matter of personal pride that person will want the shoes to be clean - if not, what is the point in wearing leather shoes? Going around with dirty leather shoes is comparable to walking around with mustard and ketchup stains on a white shirt. So if you visit the mercados, there will often be dedicated shoe shine stands. And in many areas there will be ubiquitous niños pobres in the parques and plazas, begging to shine shoes. In any case, there will be the expecation to tip for services provided. If someone can afford to wear leather shoes, that person should be able to tip.
Or, consider the restroom? There will often be a person who is handing out towels or providing other minor services. There is an expectation of a tip for the service provided.
Since the government in most of these states doesn't provide welfare, the burden of providing subsistence falls back to those who are wealthy. And while I don't think of myself as wealthy in the US, once I venture outside the US that picture changes. And I adjust my behavior accordingly, trying to adapt my behavior to the locale where I am, and not expecting the local culture to adapt to my customs. To gringos, it seems as if there is always someone with a handout, expecting to be paid for some trivia activity. But in the prism of local norms, that might not be the case at all.
That's how I see it, and I said my piece. You can agree, disagree, or find some space in between. Each of us needs to find the spot where we are comfortable, and be content there.