Why does the Yahoo search engine allow advertisers to redirect your search.....?

to something you have absolutely no interest in whatsoever? I mean I am searching for lyrics and I get redirected to prostrate medication ads and mental health publications. Then when you hit the back on you are redirected to the same site again. You actually have to discontinue your search to go back to your home page and start again. Why would the server allow such antics when it is the consumer who makes the final decision as to whom he will give his business?