“Isn’t the Halacha that you’re kona an item as soon as you pick it up, even before you pay for it?”
Not if the present owner does not consent. Furthermore, in a store the kinyan is made through money. That’s the gemiras daas. You have never seen a kinyan otherwise.
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“It’s in the owner’s best interest to let people snack on their purchases while they shop…”
While the store owner would not want a screaming kid, s/he also does not want shoplifting. The item does not belong to the customer until payment. A hungry kid is the parent’s responsibility, not the store’s.
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I find this moreh heter bothersome. If there was a safek of a treif ingredient, you would not touch it, but consuming stolen goods is just as bad – if not worse. I recently heard in the name of Rav Yaakov Kaminetzky [sp?] that one reason that kids go OTD is that they ate food acquired through dishonest means, basically gezel.