More purses are stolen by right handed purse snatchers, so right handedness causes purse snatching.
you got it backwards. It is expected that most purse snatching will be done by right handed people, since there is more of them. Just like you would expect most crimes to be perpetrated by a white people (since there is more of them than blacks).
In reality the opposite is true, so it is very reasonable to question/investigate a possible causality.
Your example should be:
"More purses are stolen by LEFT handed purse snatchers, so LEFT handedness can possibly cause purse snatching."
It is unreasonable and irresponsible to outright dismiss a possibility of a causal like, like you do.
Also, it would be perfectly reasonable for the cops to concentrate the searches on LEFT handed people.