Do you really think that Microsoft would...
1) Pretend that no one would notice.
2) Do a risk to reward analysis, where they would gladly pay out 7 billion dollars if caught (over a few million extra IE installs).
3) Implement a fix within one day after notice, and offer to extend this ballot on their brand new OS for another 15 months.
The simpler explanation is that due to the nature of the ballot, their test Suite did not include the browser ballot in it, and was delegated to someone in the legal department that filed if away and forgot about it.
This ONLY affected 28 million PCs. That's a very small fraction of the user-base. And considering IE's market share, this bug would have only caused less than 1/2 of that number of non-installs of other-browsers... As afterwards, anyone could have just downloaded what they were looking for anyways.
Also note that for 15 months NO ONE (well, just a few people apparently) contacted MS or the EU Commission about not seeing the browser ballot. I guess the browser ballot is as important as you might think it is to the people of EU.