Because without a rule that states a cleric or paladin can get excommunicated, it would be unfair to players to play their characters or even chose to play those classes, if the DM suddenly and for the first time in his world's history, apply a more harsh punishment directly to that player from out of the blue.
Are you talking about punishing the player, or the PC?
And do I need a rule that states the GM may build a world with taxation as part of it, in order for it to be fair to confront the player of a trading PC with an NPC demanding the payment of taxes or tolls?
Divine agency is the key difference.
Whom is the paladin taking a vow to? To whom is he making a solemn Oath? His god.
What is the difference in this respect between a paladin, and (say) a rogue who makes a promise to the god of portals that s/he will pay money to that god's shrine if only the one door standing in the way between death and escape is not locked?
And why is your game apparently set up so that players have no reason to want to play their characters in the way that they promised to?
It's a contract, nobody is forcing you to play a paladin or a cleric, but if you do, you are supposed to act according to your church or your god's wishes,
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It would be beyond irresponsible to not take a direct hand in that.
I don't see any contract.
Within the fiction of the gameworld, I don't see any contract. There is worship, and devotion, and inspiration, and charisma. But I don't see the relationship between a paladin (or traditional cleric, for that matter) and the divinity as remotely contractual.
Within the play of the game, there isn't a contract either. Perhaps there is a promise (express or implied) by the player to play his/her PC in a particular way. If the player isn't keeping that promise, why not? And why do I need game rules to deal with that? (Chess doesn't need rules to tell me what to do if my opponent tips the board over in a fit of pique!)
Players often say they want to play a holy knight who helps the poor and is brave, but when the time comes, fail to do that and act selfish and cowardly like any knave
I have never encountered a player like this.
If this is frequent in your experience, why do you think that is? And what evidence is there that giving the GM the power to strip away class abilities will solve the problem?
Without any type of mechanical consequences... I probably would (it's why i don't play paladins in 4e), if it was convenient... saved my character's life... gave me an advantage during a challenge and so on I'd break my oath. I mean don't get me wrong I'd play the paladin role when it was easy but why would I keep it up when and if I didn't have to?
My response to this is similar to [MENTION=22779]Hussar[/MENTION]'s - ie one of surprise and a degree of puzzlement.
It's a game. Whatever action declaration you make for your PC (at least within very broad limits), you get to keep playing the game. So why declare actions for your PC that break the oath? What's the rationale?
I'm having a fairly hard time envisaging the games that you and [MENTION=6776483]DDNFan[/MENTION] seem to have in mind. You seem to think it's fun to play a paladin, but also think that you won't actually
play a paladin unless the GM is threatening to take away your gamepiece if you don't stick to the oath. I'm not going to say that that is contradictory, but it's not very clear to me what the game looks like that would make sense of this motivational structure.