Ask HN: Is Ad-hijacking or Ad-injecting legal? Or just frowned upon?

I see a lot of hotspots around these days offering free or paid for usage. Is it legal to hijack ads on websites and replace them? I've looked around and have only found this article (http://www.networkworld.com/news/2008/041608-isps-meddled-with-their-customers.html) which doesn't mention any legal action. Does HN have any other information to support that it is legal/illegal for hotspots that offer internet access to hijack or inject ads?