Deployed by stalls in high foot-traffic areas, these are supposed to discourage sticky-fingered passerby from getting any bright ideas.  More tolerable than the annoying “Paid” stickers/tape that stores put on the your goods if you don’t want a plastic bag (now technically illegal here anyways). Proof of purchase and security are two separate but related issues, with security trumping most other things for street-level enterprises – the likelihood of catching a thief who is already in the street is slim to none, but the strong collaborative networks of vendor clusters that work a particular area could both raise overall security through mutual vigilance of each others’ stalls/inventory.  Cheaper than printing receipts, buying “Paid” stickers, or installing CCTV is your neighbor/competitor/ally keeping their eye on your stuff with the understanding that competition is competition, but fellow vendors come before thieves.