The commands God gave to Ancient Israel are normally divided into three kinds: moral, civil and ceremonial. This question and this question have more information. This one, and any of the others invoking the death penalty, certainly fall into the 'civil' category. The state of Israel (loosely speaking) would have the authority to impose the death penalty for a variety of offences, as many states do now.
Christians are no longer expected to obey "the law", which includes this kind of law, though most would say that they are expected to follow the 'moral laws', of which this is not one. That's not to say that 'witchcraft' is now OK,and God approves of it, but the authority to punish with a specific penalty doesn't apply to Christians in these times.
Because 'the state' is given power to punish an offence that does not give a random mob of people the right to harass someone.