I live in Brazil, and an acquaintance of mine is involved in favela tourism, though I’ve never taken a tour myself. (He has a second job to cover his living costs; he doesn’t make any money from the tours AFAIK, as he only helps promote them as part of an aid NGO he created) A lot of people jump to conclusions in assuming that it’s the tour guides and/or travel companies that are making money off this and that the residents aren’t involved. Maybe that’s the case elsewhere, but here in Rio de Janeiro they are typically (won’t say all, because I’m not familiar with every single one of them) organised on a community basis; that is, the communities of the poor neighbourhoods get together and allow, promote, and assist the favela tourism themselves. This was even the case years back when the favelas were run by drug barons, but even more so now since they’ve been kicked out.
In a typical favela tour, the guide is a resident of the slum itself, or is otherwise heavily invested in the community. The path of the tour is agreed among the residents – that is, they try not to impose on anyone who doesn’t want to be involved. People who help out (show their homes, etc.) get a share of the benefits. Crucially, vendors – souvenir sellers, street drinks vendors, restaurants, etc. – are ALL locals, and their income remains in the community (as opposed to tourism on Copacabana beach, for example, where most restaurants are franchises of major corporations, many are foreign-owned, all are taxed heavily, and basically all the proprietors are already wealthy and often abroad).
As for the travel agencies – many favela tours are sold on the street or advertised directly in hotels, etc. I suppose some are promoted in packages where the agencies take a percentage, but surely no more than with any other third-party item they sell.
So I can’t comment on the situation in India, South Africa, or the slums of first-world countries, but here in Rio, favela tourism is seen as something very positive by those involved. Sure, mostly the local attitudes are “why in the world do these weird gringos want to stare at destitute families in dilapidated shacks when they could be sipping overpriced cocktails on the beach?”, but it’s a way for money to get into the hands of the poorest and most disadvantaged in one of the world’s most economically unequal countries.