They wouldn't necessarily have received useful data from it. Consider what happens if you spoof the sender IP and port in the first packet of a TCP handshake:

They wouldn't necessarily have received useful data from it. Consider what happens if you spoof the sender IP and port in the first packet of a TCP handshake: the recipient will send a response to the spoofed IP, making it look like they are the bad guys. Someone isn't filtering Martians properly, or those spoofed packets would have been filtered before they reached you.