Prague PID Supplemenatry Charge

I am currently visiting Prague and wanted to try the public transport. I bought a ticket and enterd a Tram. As soon as the Tram began moving two employees asked for my ticket I showed them. However it was not valid for another 30 seconds so they insisted i pay 1000 czk. I tried arguing with them, no chance. I personally feel like they could have let it slide. I had bought a ticket and it was my first time using the tram. Are they always this strict? I am definitly not going to use the tram again…