I think your last paragraph clearly ilustrates both the scope imposed limitations of the google mail service and its usability.
Give it a thought. To start with, the advertisement was targeted.
If they choose to reply to me, I will see none of the ads they saw. Their email service didn't send me (as a non gmail user) any kind of ads. Since I didn't subscribe to their service, why should I get targeted for their ads?
This in my view is correct.
Second, the email messages contents sent back and forth are ad free. There are no extra lines in the bottom (or top or middle) of the messages. There is no indirect advertisement. Throughout the whole conversation I can rest assured I will never be targeted by gmail advertisement system.
This is also a correct way of doing it, as I see it.
Now... The most important bit. The ad results for the gmail user to whom I replied.
The fact he got 1 out of 10 ads not directly related to the brand being discussed (note it was not Rebook, it was a shoe shop ad), serves everyone. It serves my customer because he will have the opportunity to compare and choose. It serves me because the most of the results and the top results where related to my brand. That is what I payed Gmail to do.
I cannot honestly expect to be the sole result of google ads within the context of my business. The fact that someone named my brand on a message and my brand got 9 out of 10 results and they were the top ones, is much much better than what I get with contracts signed with TV stations and that cost much more. Take for instance a look at how many different brand shampoo commercials you can see in one ad break alone.
I advise you to read the contract you sign with Gmail when purchasing ad space through their system.