It's all off-the-shelf electronics and standard protocols. Venue provides some wifi with a "Ticketbastard" SSID (or whatever) at entry points, and the COTS-built barcode-validating devices use that. Easy-peasy.
They might also provide other wireless networks for other purposes (definitely for vendors [$$$], but perhaps also for regular house staff, touring staff, and maybe even the guests who pay for it all!), but they'll all be under the venue's control and coordination: Other than the odd personal hotspot that wanders in, there's not necessarily any meaningful outside interference on 2.4/5GHz wifi bands in a big venue.
It's pretty easy to make short-range wifi work reliably in that kind of RF environment, such as the chokepoints where tickets are validated. (Modern apartment dwellers will have worse interference problems than that.)
There’s actually a ton of interference in the 2.4 GHz space, especially at venues like outdoor festivals. However your solution does work. I work at a festival that provides a WiFi network and an Ethernet drop for the ticket scanners. We have to use multiple APs to cover the main entrance area, but it’s feasible.
I was thinking more along the lines of a stadium crowd than an outdoor festival, but yes: I agree. I've had miserable luck with 2.4GHz stuff in festival environments where people camp out for a few days. :)
I don't pay very much attention to the ticket-scanning devices while I'm getting into a big show (which is generally a rather unpleasant experience on my side), but:
Don't they allow usage of 5GHz bands? Unlike 2.4GHz, I've had tremendous success with 5GHz bands in all kinds of environments -- including outdoor festivals.
