An ISP that modifies web pages that pass through it may well be in breach of the copyright of
the page author. They have an implicit license to redistribute the unmodified page, but I
believe do not have any license to redistribute adaptations/derived works of that page.
This presumably applies to all transcoding proxies and other non-transparent proxies too. The
only differences I can think of is that adding advertizing is for commercial advantage, and
that copyright owners may be more likely to sue someone for adding/changing/removing
advertisements than other modifications.
I must note that IANAL, and in fact I'm rather less sure about the above than most things I
might say about copyright.