If you have a Day Travelcard, you are meant to get the BZ6 ticket, rather than East Croydon, if the train doesn't call at East Croydon. But if you ask for BZ6 and are sold East Croydon, I'd suggest Southern are happy to accept such tickets on their non-stop services.
The issuing of tickets to/from named stations when you ask for a Boundary Zone location is not unique to Southern by any means.
But if anyone has evidence that Southern are deliberately selling the wrong ticket to catch people out on their express route, that is serious and needs to be dealt with appropriately, I'd suggest contacting Southern initially and then go to London Travelwatch, your MP, and the (theoretical) regulator the DfT if it persists.
The issuing of tickets to/from named stations when you ask for a Boundary Zone location is not unique to Southern by any means.
But if anyone has evidence that Southern are deliberately selling the wrong ticket to catch people out on their express route, that is serious and needs to be dealt with appropriately, I'd suggest contacting Southern initially and then go to London Travelwatch, your MP, and the (theoretical) regulator the DfT if it persists.