Brian X. Chen, for the NYT Bits blog:
After some sleuthing, Mr. Watt, who has a background in developing
Web advertising tools, realized that the quirk was not confined to
his site. The hotel’s Internet service was secretly injecting
lines of code into every page he visited, code that could allow it
to insert ads into any Web page without the knowledge of the site
visitor or the page’s creator.
Yet another reason to bring your own 3G or LTE hotspot with you when you travel.