I take it that you aren’t complaining about being saved a complete single journey charge?
I’m not familiar with Oxford Circus. Is it just one entry and one exit gateline, or does the entry gateline also do exits? If the latter then you may be able to enter, exit and enter again. You could also touch on a bus, albeit with a charge, so I realise that that option won’t appeal to you. If you end up with a here to here journey, and two maximum fares, then the helpdesk will certainly resolve that.
The reason it is there is to assist passengers who want to interchange but end up at the exit area with nowhere else to go.