please correct me, thanks a lot

Today I traveled to Walton on Thames, after that I took the train back to Clapham Junction at 9pm, then I had to transfer Overground back to Imperial Wharf.
The Overground staff checked my Oyster card on my way back to Imperial Wharf, and they told me that I have to tap in at the Clapham Junction in order to take the overground. Obviously, I didn’t and I didn’t even realize. I accepted that was my fault, but I have to say it was not 100% my fault. Why doesn’t the Overground service set up a checkpoint like most underground stations do? Or why don’t they just place a big sign which says, "PLEASE TAP HERE! OR 80 POUNDS WILL BE CHARGED! "
English is not my first language, so it is hard for me to find the small sign. The two staff didn’t charge me, which I really appreciated. I explained my situation and they finally trusted me, but it still made me so embarrassed.
This letter was not written as a complaint, but I would like to warn those people who might meet the same situation. Also, it wouldn't cost a lot of money for the Overground service to set up a checkpoint at the station, unless they are earning too much money this way.
Thanks.

Kind regards