I know this question might sound silly, but the idea that OP actually downvoted an answer because he/she didn't like it sounds sillier to me. An example here. As far as I know, OP has the ability to accept an answer. If he/she likes the answer, then simply he/she would accept it. If not, OP would just ignore the answer. Another thing, the downvote button is used when an answer is not supported by enough sources or was badly formatted, etc...If so, then is OP allowed to use the downvote button to downvote an answer he/she doesn't approve ? Shouldn't the OP use the comment's section to suggest an edit which might improve the answer instead of downvoting ?
They are free to downvote if they don't find it useful. Note that Shia and Sunni have different opinions about which hadith are sahih, so what seems like a solid hadith to yourself may be considered a false hadith to someone else.
But to be honest, I wouldn't worry about it. A decent answer that has been unfairly downvoted will almost certainly be upvoted to 0 or -1, so you get a net gain in points for unfair downvotes.
Downvoting is used to indicate that an answer is not useful; even if the OP doesn't know the answer (hence why he's asking in the first place), he is in a position to know whether any given answer is actually useful to him.
However, I find that oftentimes when the OP is downvoting an answer, it's because the question itself is poorly written (this is just a general case, I am making no judgements on the linked question); it is as much a responsibility of the OP to write a question that attracts useful answers as it is for the answers to actually be useful. If the OP is regularly downvoting answers that look to be exactly what the question is asking for, the question itself probably needs to be edited to clarify what OP actually wants in an answer (or closed as NaRQ).
For the most part, however, don't worry about it. Downvotes happen, and often for no logical (or at least logically explained) reason. What is, or is not, "useful" is very open to interpretation and personal whims.