It really does not make sense to me that why one is allowed to downvote an answer and accept it simultaneously?
How can one show disagreement and agreement simultaneously? Non-sense it is!
I agree it should most of the time (though one could envision scenarios) make little sense to accept an answer and to down-vote. However, not everything that makes little sense needs to (or even should) be prevented by the software.
If you think it is a non-sense to do it, just don't do it.