This simply isn't true. Anyone with a credit or debit card can establish a 'cash' account and buy from them. So no different from most 'mail' order places.
If you're talking about their trade counters, they don't want 'Maplin' type customers wasting their and their customer's time. Wish more nominally trade places were the same.
The original idea of this was to protect their main customers businesses by not selling direct to their *potential* customers.