And it's not only about user data, it would also expose the website to content spoofing in public wifi, which would for example allow the attacker to inject fishing content in the website.
SSL encrypts the data you're sending but it also ensures that you're communicating only with who you think you are. Without SSL you can't be confident about any of that.
That's still my point, for example you could inject your own login system "create an account to keep track of your favorite artists, or some new shiny feature". For there you can get people's personal information, potentially a password they use on other services.
An URL is something the general public will trust, if the content can be messed with you repurpose the website's reputation. I took phishing as an example but even my not-so-creative and non-expert brain can think of other things : asking for donations, propaganda, advertising, censorship, ...