At my local supermarket, they keep the baby formula locked up because people steal it to mix it with heroin.
More likely they are trading it for heroin. It's easy to resell and many jurisdictions won't prosecute theft of baby formula or other "essemtials".
They keep it locked up because the brand names are exorbitantly expensive. The reason the brand names are expensive (unlike the generic versions) is because the government (meaning us) is paying for it via giveaway programs like WIC that only authorize the expensive brands.