I must admit this does seem a reasonable question, as the scanner is only a receiving device, so can't really see any technical reason, although I do stand to be corrected.
That said, there are very obvious safety reasons. What if after take off your pilot declares an emergency for whatever reason. You will obviously be privvy to his talks with ATC and may react in a hysterical way, alerting other passengers who might also freak out, and before you know it the whole plane is in a panic. Whereas what would normally happen is a quick message from the pilot that there has been a problem and you are returning to X, followed by a normal landing and a normal disembark, or at worst a quick trip down a chute, that you learned about 3 mins before landing. Harldy enough time to get into a lather about it really.
I can understanf the desire to listen, and I could imagine that it could actually be quite interesting to a non pilot, but although there might be other more serious reasons, this one alone is good enough for me to say NO!