Is giving resources away for free allowed?

Question

Motivation: I once had one surplus card on my hand, and knew the robbers would come soon. So I gave it to someone for free, to avoid losing more cards when they hit.
I was surprised by how badly this was received. Two of the three other players (the hosts of the game) acted upset, like I had tried to cheat. I had to take the card back, get robbed the next turn and 'giving cards away for free' was banned immediately. (Oh, how I hate mid-game additions to the rules. But that's beside the point.)

So now the question: Is this really illegal in Settlers of Catan? Is it generally considered underhanded in any way? If so, why?

My reasoning was like this:

Consider players A and B.
A wants to give B a sheep.
A trades a sheep and one rock to B, in exchange for wheat.
A trades the received wheat for the (formerly his) rock.
A and B now both have the same resources they had in the beginning, only the sheep has moved from A to B.

Answer 1

Contrary to what mafutrct has stated, there is an official rule against this practice:

You may trade with another player between your turns, but only if it is his turn and he elects to trade with you. You cannot trade with the bank during another player’s turn. You may not give away cards.

(from the Almanac section of the rulebook, near the bottom of p13)

I understand that you can "effectively" give away cards by cunning sequences of "legitimate" trades, but it seems clear that this isn't really in the spirit of the game, and players should feel free to frown upon such sharp practice!

Answer 2

This question is answered in the official Settlers of Catan FAQ:

Trade - Can I give away resources or buy services with them, for example, to avoid being bothered by the robber?

No. On Catan, a trade always involves a give and take of resources. Consequently, Catan’s trade law does not contain a “trade” of something for nothing or for immaterial goods such as services.

Although you could get around this by doing the 2:1 then 1:1 trades, it violates the spirit of the rules and would be looked down upon by your fellow players.

Answer 3

There (edit) is an 'official' rule against this idea, since one player is unfairly favored over the others, and it may create trouble in practice.

In my games, we usually avoid this issue by trading 2:1 (with a player) to a resource you need anyway. If you got any 2:1 ports to reduce your number of cards, use those instead as it is fair to everyone.

Example: Assume you intend to build a city, but have two extra sheep you don't need and lack a wheat. Your number of cards adds up to 8. Simply trade the sheep 2:1 with someone to wheat and you'll not only avoid the robber, you'll also make progress towards your goal.

Either way, this pretty much only applies in the early game when you're unlikely to get many resources per turn. Later, when you've already got a number of towns built, you'll get new cards almost every turn so giving away one of them is less important.