Here's the logic, in some religions (Judaism, Christianity), necromancy is considered a sin (while in some other religions it's a perfectly normal part of life, but that's a topic for another time). Now, does ghost hunting count as necromancy? I've always thought of necromancy as summoning and seances, Ouija boards even, trying to get answers from the spirits. But trying to communicate with spirits is part of ghost hunting, isn't it? The attempt to capture EVPs and other responses.
But note, there are Jewish and Christian ghost hunters and psychic mediums (which is pretty much the modern term for necromancer), famous ones even (like the Warrens). So, is there a key difference? Is the intent and goal of the action reason enough? Because not everybody thinks so, or that the intent/goal is even different. You're still looking for the spirit to give you information you didn't have before.
Or maybe it's possible to ghost hunt without committing necromancy? To resist the temptation of asking for signs and communications, and simply record the evening without an EVP session?
Of course, the next logical question is should they engage in blessings and exorcisms? It's hardly scientific, but neither is asking a ghost to turn on a loose flashlight.
Further Reading and Resources:
“Christians and Ghost Hunting”
Ministry Matters (last modified 2012-10-12)
Christian Ghost Hunting (discussion):http://theparanormalchurch.ning.com/forum/topics/christian-ghost-hunting
“Dangers of Ghost Hunting”
Spectral Intelligence Solutions (last modified 2013-01-01)
Krulos, Tea. Monster Hunters: On the Trail with Ghost Hunters, Bigfooters, Ufologists, and Other Paranormal Investigators. Chicago: Review Press, 2015.