Actually it is a test, and how well you answer it is directly responsible to how your business will or won't succeed.

Google has worked out their algorithms for processing the incoming data and generating an answer. Bing has apparently used Google's answer as a comparison to whether they are getting it right or not and when not, subbing in the other answer.

While in business this may not be illegal, is it still very much 'Not knowing the correct answer to the test'.

No, it's a product. Come on. Every product is a "test" of the market so in that one definition of "test", yes. But not in an academic sense, which is what you meant. Because only in an academic sense does the concept of "cheating" exist.

Look at it like a machine learning problem. If you slavishly match the test set you are doing something called overfitting. Your performance on the rest of the web will decrease, because you inherit bias from the test set.