Its slightly different at an airport where you have to pass through security, to a supermarket where you've paid for your goods but Sainsbury's/Tescos/whoever just scrimp on staff costs and make life difficult for everyone. They don't pay me to scan my shopping, so why should I prove to a barcode scanner that I'm in possession of a receipt?
I know that supermarket analogies never go down particularly well on these forums but I see many similarities between self-scan shopping and train tickets. The stations on my local line have all been completely unstaffed since way back in BR days. These days many people have 'self serviced' via a ticket vending machine, ticket on departure, print at home, barcode on mobile phone, bought one from a mate down the pub who's an expert with Photoshop or whatever rather than using a staffed booking office.
There are many benefits to the shopper from self-scanning and as a regular Sainsbury's customer I was an early adopter. Altogether a handier experience in my view. And scanning a receipt is a small penalty to pay in order to deter somebody else from waltzing through with bulging bags of stolen spirit bottles, etc. that law-abiding customers end up having to pay for in generally higher prices.
Without getting bogged down in the precise legal differences between a railway ticket and a 'receipt' a straightforward scan on exit in either case hardly seems like a big deal.
At the risk of drawing attention common methods of fare evasion; people will often seek to avoid having their 'ticket' scanned or presented at a barrier so that it can be fraudulently refunded, re-used, sold on or whatever (assuming that isn't a fake or has no supporting discount card or age entitlement anyway). Barging through or tailgating sets a lot of alarm bells ringing in rail staffs' minds.