As viraptor said, putting code into a public repository (e.g., debian packages); that way there should be a paper trail if the code is modified.
Beyond that, there's a simple matter of advertising: "Share your terminal (read-only)" may mislead some people about what is happening. A more accurate description would be "Give us control of your terminal (we promise we'll only let other people read it, not write anything)".
Imagine if this principle were applied to other software products. "Click here to download the Office installer, which will gain full admin-level access to your machine. We promise to only use that to install Office."
The only thing sent to shellshare's servers is the text in your bash terminal. There's no return channel for the servers to send commands back to the computer.
You could argue that you're giving control of the terminal because you're running a third-party executable, but that's the same for any executable you run.
Beyond that, there's a simple matter of advertising: "Share your terminal (read-only)" may mislead some people about what is happening. A more accurate description would be "Give us control of your terminal (we promise we'll only let other people read it, not write anything)".