graeme_p - 5:45 pm on Aug 17, 2013 (gmt 0)
Why is it inethical? They are not selling your info to third parties, it is not even being looked at by human beings. It is subjected to a little additional processing by Google's servers, which have to process the email anyway to deliver it, store it, filter spam etc.
It is a lot better than the TOS for Windows that you do not get to read until AFTER you have bought a computer with Windows on it.
It is not a secret. The ads appear in Gmail Google can hardly be pretending they do not place ads. It is a perfectly reasonable practice, no different from using search keywords to place ads on Google search.
What do you expect Google to do? Not let users sign up until they have completed an exam to ensure they have read the TOS?