Your "control variable" only matters if the hypothesis they are trying to prove is "Bing special-cases Google."
That is not the hypothesis. The hypothesis is "Bing has results in its index that it could not have gotten in any other way than from Google search results." Their experiment does indeed confirm that hypothesis.
The problem is, the intentionally ambiguous and misleading wording Google has been using implies the test was "Bing special-cases Google". This is why I'm not buying any of this. Google is smart enough to use precise language when they want to, and apparently not to when they want to.
If your analysis is correct, you should be able to explain a scenario under which, given Google's experiment, Bing's result for "hiybbprqag" came from somewhere other than Google.
It came from the Bing toolbar tracking the user browsing. Yes, obviously "hiybbprqag" came from Google. But that's because they only tested it on Google.
They never tested the fact that it could have come from any other website as well. Thus, they can't conclude that Bing is copying Google or whether its copying the user's browsing behavior.
> They never tested the fact that it could have come from any other website as well.
It doesn't matter, Google is only complaining about what Bing has copied from Google. What Bing copies from other sites is between them and the other site.
It means that they have falsely arrived at a conclusion due to positive bias. They might be right, but they haven't proven it sufficiently.
The inverse of the conclusions of their experiment are also incorrectly assumed (Google results, using IE8/Bing toolbar, make Bing results != Bing results, when using the IE8/Bing toolbar, are from Google).
That is not the hypothesis. The hypothesis is "Bing has results in its index that it could not have gotten in any other way than from Google search results." Their experiment does indeed confirm that hypothesis.