Does lava-filled moats served as a protection for my house be considered as non-PVP killing machine? Example:
If it's very obviously there then it's not considered a player trap, they become illegal once they're concealed in some way. I don't really see the need for a lava moat in terms of protection, you can just use /lock to protect doors, chests, etc. Lava is really easy to get around unless it's protected by a town.
Is that the reason why the town owners not wanted to be banned for killing in non-PVP zone by that manner?
It's not a trap, it's a perfectly visible landscaping feature. A trap is something intended to kill people.