The block giving rule is not listed in the check giving it to rules.
The block giving rule does nothing when giving the stone to Peter.
Inform has the action all coded in, but blocks it with that rule. The former example removes the rule entirely and thus allows all giving to proceed normally. The second makes an exception for this one particular action.
The reason to use this instead of an Instead rule is that this allows the action to go through the usual machinery, allowing other rules (e.g. you might have an “After giving the magic ring to someone:” rule for a special effect) to affect it, and marks it as “Succeeded” instead of “Failed” (which can be important for Unsuccessful Attempt rule output).