The person selling drugs at a festival is extremely far removed from the illicit manufacturer. They almost certainly have no idea the quality of drugs they sell or what’s really in them.
When it comes to festivals, the persons walking about selling hand to hand are some distance removed, the person who decided to sell at Glastonbury (for example) or other festivals aquired supply by the van load, acquired a crew of runners, pushed back against others wanting target the same festival, and is considerably closer to the upstream source and has a damn good idea of the quality of the bulk amount they sourced for that market.