How come teams that have traded players with RS don't have the opportunity to receive that portion of salary relief for LTIR? For example with Matt Murray, what are the downsides with Ottawa having the chance to utilize the 25% RS as LTIR relief?

I was wondering the same thing. When they trade someone, while retaining salary, if the receiving team subsequently buys the player out, the sending team takes its portion of that hit. Dion Phaneuf is an example of that - again for Ottawa.

It seems to me that what is good for the goose is good for the gander.