I think in WPA-PSK each client also has unique key (called PTK). this key is derived from PSK+2 random numbers and MAC of both computers. this always happens when a client connect to AP (this is why you need to capture the handshake to decrypt packets on monitor mode). So by your logic this wouldnt be possible to, but the fact is that it works in my experience and Im almost sure that it works in wpa-enterprise too, as i think i remember testing.
Now you put me thinking about it, and you get me confused about "why it works?"
I think its because the packets before reach you are sent to router first, then are reencypted and sent to your ip using the encryption your wireless card knows..
its somethig like this i think its happening
victim->router->internet and vice versa
victim->router->yourpc->router->internet and vice versa