When a Stack Exchange employee invalidates votes in cases of voting fraud any reputation earned from acceptance of answers is removed, but the actual acceptance stays. This results in a the answer staying pinned to the top, and having a very notable graphic indicating it is "the best answer" even though the acceptance was deemed fraudulent.
While it is true that most mods, in cases such as this, frequently tend to delete these answers (and they frequently merit deletion in their own right, regardless of the voting fraud) there are likely to be some number of cases where the answers don't merit deletion or where a mod or employee doesn't find all of these answers to delete them.
By having the (automatic or SE employee triggered) vote reversal script also undo acceptances it ensures that these fraudulent answers aren't given undue attention without giving moderators fined grained control over the acceptance of answers.
(Note that this proposal was originally mentioned by Brad in this comment, along with some related discussion.)