People don't like being duped, even if it's for their own good. You're likely to get more "f--k you's" than "thank you's".
If you want to teach someone a lesson, you don't start out by telling them they are stupid.
To create some real, lasting value you could have created the app and then said something like, "Hey everybody! I made this fake marketing web app that will steal your information and show you how it's done step-by-step. If you want to see how web scams are done, follow this link: ..." Then make some fake Google accounts for people to use (instead of their own).
"... you have to wonder if the collective value of the information given up does not exceed greatly the value of the goods they are shipping."
+1 for that sentiment. If your info wasn't more valuable, they wouldn't be doing it.