The reason it's against the rules is not because it's immoral, but because it is impossible to control. Say you are lending an item for 3k. You have two options here. One, you can collect your 3k before you lend out the item. In this case, you could just pocket the money and walk away, as the old "trust trade" scam went. In the second case, you lend the item for a select period of time, and then collect. This time, the other player can just walk away with the lent item and you are out of luck until the time period is over. If you choose to lend until one player logs out, then you are back to the first situation of you walking away, or, in this case, logging out, and you pocket the profit without any penalty.
It's against the rules because of the possibility of scammers promising to lend an item just so they can get rich and it would be a nightmare for customer support to sort through the valid complaints of scamming.
It's not against the rules, or the very numerous threads in Armor&Weapons that have "renting X item $$$K an hour"
are not locked. Also, the sticky for lending items specifically says that thread is not for Renting, or Charging people to borrow an item, yet there are tons of posts advertising borrowing X godsword $$K/hour and vice verca. They're not hidden.