Your question is pertinent: If we should not use the Ouija board to try to contact spirits, as many paranormal researchers warn, why is it ok to try to contact them with voice recorders (for EVP), ghost boxes, dowsing rods or any of the other tools that ghost hunters use? It seems that any unwanted entities could just as easily influence these tools as they could the Ouija board.
In a very broad view, we cannot really answer these questions because we don't know with any certainty what makes a Ouija board move, how EVP are recorded, or who or what is influencing any of the mentioned devices... or how. Are they influenced by disembodied spirits -- good, neutral and evil -- or is it just our subconscious at work? We don't really know.
The Ouija board is not inherently evil; I'm sure there are just as many (probably more) good and neutral experiences with the Ouija as there are negative ones. It's just the negative experiences that get the attention.
However, I think the intention of the ghost hunters may have a lot to do with the performance of these tools and the results they deliver. Many people use the Ouija board with the expectation that something bad, negative or evil will happen, and they often get what they expect. The same may be true of the other tools. I doubt that many ghost hunters use dowsing rods with the expectation that something bad will happen with them. The same is usually true with the use of voice recorders and ghost boxes: there's not that same Ouija expectation that something evil is going to come across. Although negative sounding things have been captured by these devices, they don't carry that same "weight" as the Ouija.
Bottom line: the difference, I suspect, has a lot to do with the intentions and expectations of our conscious and unconscious minds. When you think about it, if there are spirits out there communicating with us, it is really their consciousness contacting our consciousness. The tools are just tools, and our minds interpret the meaning of their sounds and movement.