If someone searches for "project management software" and then is shown ads from whoever bought ads for those terms, I think everyone would agree that feels fair. If someone clicks your ad, you are happy to pay because this is a fresh new lead delivered into your lap.
What rankles in the Basecamp example (not saying it rises to the level of antitrust, though) is that the user is specifically searching for Basecamp. Basecamp has already "earned" this lead though their other marketing spend, or word of mouth, or whatever. However, if they don't pay Google, they risk losing this lead to competitors sniping the "Basecamp" term. In a way, Google is selling Basecamp's leads to competitors (contrary to what the user actually wants!) using their "monopoly" in search.
The user wants the best product for their needs. If Basecamp only survives because its users don’t know about alternatives, then it’s not a very good business. This is actual competition: building the best product and letting the customers decide what they prefer. They already know about Basecamp in this scenario so seeing ads for other products will only let them make a more informed choice. The competitors, after all, are betting their advertising budget on the opinion that their product is better. It’s not free.
What rankles in the Basecamp example (not saying it rises to the level of antitrust, though) is that the user is specifically searching for Basecamp. Basecamp has already "earned" this lead though their other marketing spend, or word of mouth, or whatever. However, if they don't pay Google, they risk losing this lead to competitors sniping the "Basecamp" term. In a way, Google is selling Basecamp's leads to competitors (contrary to what the user actually wants!) using their "monopoly" in search.