People who dine at restaurants that likely commit wage theft might tip for moral reasons, but to me, it makes more sense to boycott those restaurants and communicate to managers and owners the reason why.
Workers being paid substantially less than their labor is worth by their employer is wrong.
Tipping just about doesn't in Australia, where we pay a reasonable minimum wage ($17.29 per hour for adults in full-time employment). You occasionally find people will tip a taxi driver or bartender, but by and large it's just not done.
Employer: Likes tipping
Employee: Likes tipping
However, overall I think I'd prefer to see the end of tipping culture in America. There are too many downsides to it to justify the upside (which is effectively a kind of market segmentation / consumer surplus capture). For one, a lot of places allow different minimum wages for tipped positions, which is a very bad practice in my opinion and defeats the purpose of tipping. For another, tipping makes harassment (especially sexual harassment) of waitstaff by patrons a lot harder to combat. It also skews the income distribution between the waitstaff and the back of house staff. All of these things are bad verging on extremely bad. And they could be substantially improved by eliminating tipping. So even though I'll miss being able to be generous with a tip when I feel like it, I think it's time for the practice to be put out to pasture.
But when it's clear what to do, like in restaurants and cabs, I'm happy to tip a percentage.