I'll need a source for that claim since it sounds blatantly made up. Aggregate/gravel costs $10/t., So, to get enough money for a snack, an addict would need to do an ungodly amount of work, digging up large stretches of highway and breaking it up hundreds of meters, or even a few kilometers.
The cost-to-benefit ratio just breaks down. You spend more calories making just $10. That's why vandals go for catalytic converters, copper, and aluminum. These are expensive metals that have an attractive labor-to-payoff ratio. Gravel is abundant in the countryside and no matter how poor or addicted a person is, the labor-to-payoff ratio makes no sense.
Also they're not selling the gravel, they're mostly just using it to build their homes. The economics are also very different. There's this idea that you're a dummy for buying anything. Like my sister would buy fruit at the market and her neighbors would laugh at her "don't you know you can just pick fruit on the tree??"
I'm just assessing your claim logically and it still doesn't make sense. For locals who live in the countryside, surrounded by fruit trees, it doesn't make sense paying for fruit when you can just walk over to your neighbor's tree, collect as much as you want and walk away.
The tradeoff is that your neighbor can also do to the same to your tree when it's ripe. This is not an African or third-world thing. In any farming community, whether in the US, Europe, or China, some even pick fallen fruit and put them in baskets for passersby; if you don't, it decays where it fell.
Given this is a low-income, agrarian community, this makes sense. It'd be unwise to spend money buying fruits when trees are (maybe) communally owned. So, the locals are not shortsighted ghouls who believe in just taking what they want. They see your sister as a member of their community and they're telling her, "Gee! help yourself. Stop acting like an outsider, lol."
Regarding gravel: anyone (relatively) wealthy enough to embark on building a concrete home in a third-world country won't have the energy/time to break up concrete to harvest aggregate. And when you break it up, you can't extract it perfectly. This is concrete we're talking about, no? So, it'll be clumpy. Again, the effort-to-outcome ratio is insane. Doing your boring day job and buying gravel from bulk suppliers is more reasonable.
My sister was a peace corps volunteer in guinea and the development workers told her this. On a related note a huge problem in Africa is that a lot of things happen and no reputable news organization bothers to cover it (see Lolianda in Tanzania or early Tigray in Ethiopia)
The cost-to-benefit ratio just breaks down. You spend more calories making just $10. That's why vandals go for catalytic converters, copper, and aluminum. These are expensive metals that have an attractive labor-to-payoff ratio. Gravel is abundant in the countryside and no matter how poor or addicted a person is, the labor-to-payoff ratio makes no sense.