I'll hazard a guess that Google is against keyword stuffing because, although your intentions are perfectly logical, you are trying to trick Mediabot (not to mention Googlebot for search).

I've said it before and I'll say it again. Google should utilize the meta keywords tag to let webmasters give hints as to what ads to display.

I thought the meta description did that?

solobrian, if a few small keyowrds were enough to do the trick, you might as well do the same, with more style. You might for example:

1) place a sidebox on every page with things like "are you looking for cheap flight for Elbonia?. These are tips that might interest you." Rotate these phrases to serve different variants

2) Do specific pages on "Air fare to Elbonia", "Cheap flights to Elbonia" .... Link to those pages from all the pages.

3) Add a paragraph to the pages itself, about the subject of flighing to the place.

In any case, the keywords will be there, but without being low-level keyword stuffing.

they constantly write emails asking me where I buy tickets

Yep, gotta' agree with frox. Your users are telling you exactly what content they're interested in. Use the feedback to develop new pages. You might even be able to make a few affiliate dollars with it.

What the original poster mentions having done doesn't violate anything I know of, and is in the best interests of everyone concerned.

Give your visitors what they want.

It *might*, however, affect how you show up in the regular SERPS.

This is from Google's Adsense Policies:

Webmaster Guidelines
In addition to the standards above, AdSense participants are required to adhere to the webmaster guidelines posted at [google.com...] Some relevant items from the guidelines are included below for your reference:

Do not load pages with irrelevant or excessive key words.

So I'd be careful just how many keywords you add and where.

Additionally I thought Frox made some great suggestions for writing some targeted content to improve ad relevancy.

irrelevant or excessive. Give it the smell test. I had a number of pages on one of my sites that got either badly targeted ads or no ads at all. Dropping a few lines of relevant text into the equation did the trick quite nicely.

My experience is that you can dramatically alter the ads by changing or adding a few words on a page.

Synonyms can be very helpful. So can simply rephrasing a comment or statement.

Not long ago when targeting went wacky on a page where it had been spot on for over a year I exchanged the word agent for professional with good results.

Actual keyword stuffing probably isn't needed.

And there is always the Adsense feature that let's you tell them to focus on or ignore certain text on a page.