Although your edit was indeed perfectly valid, and turned an objectively absurd question into a valid one, I, for one, understand the reasoning behind the rejection.
The OP clearly doesn't know the first thing about encryption and/or compression. Somebody taking the time to point out the differences between the two might be more helpful to the OP, and the SO community.
If I were confronted with your edit however, I would have approved it, because the original format will lead to tons of down-votes, discussions in comments, and irritation... so that "ideal answer" I mentioned earlier will never be posted anyway.
In the end, it is quite subjective... I have rejected valid edits because they actually answered (part of) the original question, or corrected an error in the code that could have been a typo, but could just as well have been the problem itself...
Bottom line: your edit was valid, but sometimes a valid edit might reduce the chance of fetching a great answer. though it is my personal opinion that this is not the case here