A problem I see with moderators being completely honest with people filing complaints is that people leak information. Just because that information is said between two people, doesnt mean the entire site wouldnt have the potential to know about it within days. Friends tell their close friends, who have other close friends, who tell somebody they wrongly trust and now everybody knows.
I think the mods do the best they can with what they're provided. I do however think a better answer could be provided as opposed to, "There is no evidence to suspect foul play." There's always evidence in any case, its just a matter of how well it convinces.
As I said earlier, I think a more appropriate response would be something along the lines of, "Your concern will be investigated." That doesnt state a bias to one side or the other, and it confirms to the player that the moderators have their backs. If a player were to then follow that inquiry up with a question like, "What did you guys find?". I think the proper response to that would be, "All case investigations are kept confidential. If we find reasonable grounds for a banning, it will be done."
While both statements are trying to say the same thing, one sends a different message than the other, and makes the player feel like he isnt protected in my opinion. I could be wrong, but I feel like the response I listed confirms that the investigation took place, and also assures to the player if something is found out of place, it will be taken care of. Even though we know the mods have our best interests in mind, when a person of authority doesnt express himself correctly, it causes confusion among the community(this topic).