When Time Warner did the same thing on my connection, they actually returned the RCODE as NXDOMAIN (implying a failure) along with the A records for the advert page. Resolvers which properly/strictly adhere to the RFC would treat the lookup as a failure, which means that for spam purposes this probably wouldn't have caused an issue. My guess is that web browsers aren't quite as concerned with a strict interpretation of the standards, since they want the users to get to the web site they're looking for under even the strangest of circumstances.
In either case, it's still a shady move by the ISP. At least they provide opt-out, which I guess is better than nothing.