Penalised for not having ticket while there is no

I weekly catch the Metrorail train at Dallas station, Lambton Gardens, where there is no ticketing office. When I get to my dstination station, I always buy the ticket from the ticket office at that particular station. |The tellers never give me problems as they always seem to know there is no ticketing office at Dallas. On the 5th March 2015, I caught the train to Doornfontein station and upon my arrival I approached the ticket vendor at the turnstiles to purchase my ticket. He informed me I have to pay a R46.50 penalty for not having a ticket. I explained to him the reason and the history stated above. He would not listen, but threatened to detain me if I refused to pay. I paid the fine.