I'm not opposed to throwing in whatever we don't eat - in summer that means cups of blackberries, or carrot tops, or a corn cob that's mostly scraped clean. But they don't really need purchased treats - while those are useful when you're working with chicks and teaching them to go in the run, once they learn, they can be bribed with almost anything, like a handful of their regular feed.
In full disclosure we do buy bags of chicken treat mix sometimes but they get very small amounts, maybe 2 Tbsp a day total for 10 birds. And once in a blue moon I'll splurge and buy them a bag of imperfect veggies from the farmer's market at $1 a lb, especially if my garden isn't really yet productive. But that's less for the chickens and more because my hubby likes watching them eat.