I think you missed the point in your rush to object. What's the legal difference (IANAL) between optimizing HTML and inserting ads? In both cases X leaves the source, Y arrives at the destination. Opera does something like this for their Opera Mini browser: the content that is delivered to the browser isn't even HTML, it's some proprietary format, although the browser usually correctly renders it to what the HTML would look like. However, in case of Opera Mini, I explicitly agree to such manipulations and to accompanying technical solutions.
Once again, this may be a good move on Cotendo's part that will lower their costs and improve end user experience, but it is a dangerous one, because if ISPs and CDNs automatically receive the right to manipulate transmitted content however they please, it will certainly lead to abuse in some cases.