Donating to Open Source Projects

Almost all open source projects have an option for donating to the project. These project’s web pages are generally ad free. I was just thinking it might be a good idea for open source projects to have some advertisement on their page, people who are interested in donating to the project can do so by clicking the ads on the page. The disclaimer from the advertising agency (google, bing, etc.) says that you are not suppose to encourage people on clicking ads but if one knows the economics of web advertising they would do so to donate to the project.