to answer the original question, though:
we are surrounded by media that enforces the idea that the unhealthy stuff is somehow better. there aren't really ads on tv for homegrown tomatoes. that's how capitalism works, you know.. to sell us something that we don't need and to separate us from the idea that we could do something/make something/grow something for ourselves.
the fight for healthy food goes SO deep. evil people are trying to patent seeds in such a way to remove the autonomy from farmers.. and pass legislation that makes it difficult if not impossible for people to provide sustenance for themselves-- neighborhoods that outlaw chickens, bans on raw milk, etc.
there's a lot of money being poured into marketing the products that are processed. some of it's not even direct marketing... there's much being done behind the scenes, such as placement in the grocery store of the crap cereal at eye level for kids, for example. packaging and DIRECT advertising for children, who are incredibly susceptible to commercials. they are advertised to and the ad companies call it the "whine factor," meaning the kids will whine for the product if they make it look good enough and the parents will usually give in.
because when it comes down to it, we as parents do NOT like to deny our children things.
and again, that's part of our consumerist culture.. we are told again and again that love is expressed in the things we purchase. work out of home mamas also are told repeatedly to assuage guilt by buying stuff.
lots of times that stuff we buy turns out to be food.
i guess in the case of your well-meaning relatives, they are probably influenced by the many factors that suggest things like kids LIKE the crap stuff. kids will only EAT the crap stuff. sometimes even the commercials and tv enforce this.. think of all those ads that suggest or imply that kids won't eat broccoli or spinach. or that you have to force them to. and obviously, when given the crap stuff... it's made so that we respond to it. sugar is addictive and provides some brain chemistry altering reactions that initially seem positive. to somebody who has limited interaction with kids, it's an easy way to provide happiness that is immediate and visible!