Like everything else, it is a matter of whether the other party approves or not.
Some programs actually provide mirror sites along with the banners etc.
Others consider it theft.
I used to do it, with permission, when I used to use a lot of PPC traffic. I would send the traffic to the mirror rather than directly to the casino to prevent them from seeing where the traffic came from, so they would not use my promotion methods and cut me out of the loop.
Permission is the word of importance here. If considering any method of promotion that is not usually done and is not covered in T&Cs, always ask the program.