Warden discrimination?

I have just spent a very pleasant three weeks in Malta, and during my stay I rented a hire car from a local company.

I watched the traffic wardens from the window of an apartment I stayed in at Qawra. For several days a local's car was parked on yellow lines and the warden passed it, looked at it and drove by. One morning a hire car was parked in exactly the same spot and the warden, as he passed, stopped and put a parking ticket on the vehicle. The following morning the local's car was back and surprise, surprise, no ticket!

I mentioned this to several people I met and was told this was common practice.

Is this just another way to "rip off" tourists? If this is common practice, perhaps tourists should be warned to stay away.