It is kind of funny how most of these people complaining about Bitcoin being "cheating" propose as a "solution" the blackmail and robbing of Bitcoin users in-game.
However, the more popular concern seems to be that people can use Bitcoin as an invisible, unbreakable chest outside of the Minecraft world, unavailable to other players, in which Bitcoin users may keep their valuables.
While it is true that value in the form of Bitcoins cannot (barring hacking) be taken without a user's consent, that does not mean it cannot be stolen... Here are just a few of the ways that Bitcoins can be taken:
* Threaten to attack a town, unless Bitcoins are transferred to you.
* Attack a town, and occupy it until Bitcoins are transferred to you.
* Harass and grief a town, and demand a payment in Bitcoins to stop.
* Imprison a player, and require a payment in Bitcoins in order to release them.
* Threaten to release the coordinates of a secret location, unless a payment of Bitcoins is made.
* Pretend to engage in a Bitcoin-denominated business transaction, but take the Bitcoins and run.
* Set a large bounty of diamonds on someone, which you promise to remove if they give you their Bitcoins.
* And many more!
The seller is compensated with something that still has influence in the world but is not contained within the world and does not have to be protected in the world, unlike an in-game currency like Iron that honest people use.http://www.reddit.com/r/Civcraft/comments/sb92r/you_dont_need_bitcoins_to_store_value/
tl;dr: Using Bitcoin is cheating because it prevents me from robbing you.