As some of you guessed, I don't manually control which Google ads appear on this site. I can manually ban ads from specific domains, although that takes a few days to take effect. If anyone sees something especially irrelevant or offensive, please send me a private message with the domain name of the advertiser and I'll ad them to the ban list. Google doesn't give me any way to approve or ban them preemptively.
Choosing which ads to show is all in the hands of a huge automated system that makes Google most of its multi-billion-dollar annual revenue. Normally, Google uses its knowledge of what a page is about to serve relevant ads (like ads about fly fishing for most of this site). Sometimes, however, Google uses its prior knowledge of what it thinks you (or someone on your computer) might be interested in, such as politics, and those ads may follow you around between different sites that use Google's advertising service. Also, sometimes advertisers specifically choose certain sites in stupid ways, and I've been handed some irrelevant ads through that mechanism.
As for generating revenue from this site in general, the explanation is simple: I like money. Who doesn't? I could write a long-winded justification about the costs of this site (about $100/month for hosting, plus over $5000 up front in camera equipment and a year of full time work) but I'll admit the ads more than cover my current site expenses. A grad student stipend isn't very big, and this site helps me pay my bills through its several commercial channels (not only Google ads, but paid links in the left sidebar, and the books linked to Amazon, and the gift shop).
If you really hate seeing all ads online in general, to the extent that you're thinking of leaving sites because of them, then you could use one of the easy-to-find ad-blocking plug-ins for any major web browser. I don't use one, though. I only click useful/relevant ads anywhere online maybe two or three times a year, but I'm willing to view ads in general (albeit mentally tuning 99% of them out) in recognition of the hard work other webmasters put into their sites.
Jason Neuswanger, Ph.D.
Troutnut and salmonid ecologist