node-to-node traffic is encrypted, usually on port 9001. At the exit node, it's decrypted and sent to the final destination as standard traffic. Standard trafic with the exit node's IP address, that is.
Start posting goatse and tub girl links on slashdot and see how long it takes for your ip address to get blocked. Bonus points if you can crapflood to the point you get some pink. Then try sending some spam. I'm sure you can imagine how an exit node IP address gets put on a block list.
Tor does let exit nodes specify which ports are allowed.