When you request a website over http your ISP can intercept the request (this is how captive portals for free WiFi work, and the reason for the existence of pages like captive.apple.com to be HTTP-only). IIRC they can also insert whatever they want to on the web page, I believe there was a case a while back of ISPs doing that.
One Of The Best Games Ever Turns 25, And You Still Can't Buy It (kotaku.com) angielski