This is more than a hypothetical concern, too: back in 2011 a number of ISPs came under fire for hijaacking certain search keywords[1], Comcast has injected ads into hotspot traffic[2], and Andreas Gal has alleged that smaller search engines were buying aggregate Google search result data from ISPs trying to improve their result quality[3].

All of that is impossible with HTTPS.

1. https://www.eff.org/deeplinks/2011/07/widespread-search-hija...

2. http://arstechnica.com/tech-policy/2014/09/why-comcasts-java...

3. https://andreasgal.com/2015/03/30/data-is-at-the-heart-of-se...