It is ironic that at the top of this page which is supposedly dedicated to unmasking junk science and alternative medicine, is a Google advert entitled "I had blood pressure… " which, to my mind, is the very antithesis of what we’re trying to achieve. Go to that URL and see for yourself. It’s like trying to sell condoms at inflated (sorry for the pun) prices in the knave of a Catholic cathedral. Blerrie disgusting.
Well, look at it this way. People from this site are unlikely to buy anything from them. But they still pay if you click on the ad. So in a small way, it’s costing them… 
I suspect that neither our dear admin, bluegray V (one day he’ll have to explain the lineage of that moniker), nor the forum hosting software can exercise any control over the banner ads despatched by Google. I suspect further that these ads pay for some of the forum’s running costs. Such are the vagaries of the Internet.
One recent example I happened to notice of a monstrous clanger in the context of a sceptics’ forum exhorted its audience to click and learn how “Intelligent Design Theory showed Darwinism to be wrong!”
The one about “I Had High Blood Pressure” (www.bloodpressurenormalized.com) is a good candidate for producing exactly that which it claims to cure. ;D
'Luthon64
Yup, Google pretty much decides what is shown by analyzing keywords on the page. You can block ads from specific urls, but that will take forever and there will always be some you miss. Easier to just make 'em pay by showing their ads.
OK, did my part and clicked it.
This is common because, as bluegray said, Google uses key words. So, every rationalist blog I visit has ads for woo sites or products.
I will make an effort to click on more of them.