I childmind a girl (5) who does this and to be quite honest I think it makes her feel like she's great. Like she's being a really good helpful girl in letting me know whats going on and following the rules. The child she tells tales on is no angel but he is not challenging either. It appears as though she enjoys seeing him be 'corrected' if that makes sense.
It comes across as very spiteful but she's only 5 so as irritating as it is I always remind myself that she is not meaning to be nasty.
I think it's much more about bigging herself up, rather than putting the other child down.
I deal with it by answering her tale with a question. For example "ChildA is playing with that toy and I want it first",
Me: "Ok, did you have the toy first?",
Her: "no".
Me: "Would it be fair to take the toy from ChildA, just because you want it?"
Her:"no...."
Me: "So what could you do?"
Her: "I could ask if I can have it"
Me: "Yes that is a good idea"
Obviously it never runs that smoothly but it's an example. I'm trying to encourage her to think through her problem first before running to me, and to see it from the other child's POV.