Your IP address is included in the message to discourage abuse.
When you are sending a regular email, your IP is included as well (in the email headers) and when you are sending a regular text message, your phone number is obviously visible to the recipient. Same thing. 🙂
My plan is to include the option of having your phone number or Twitter handle appended instead by making it possible to register and associate them with your node's public key (or something to that effect). If you want to encourage me to work on this feature, feel free to contact me on Twitter and/or send me a tip!