Decline payout does not burn rewards. It returns them to the reward pool to be allocated to other rewards. There is a way to automatically burn rewards which is to set a 100% beneficiary to @null. This can't be done using the steemit.com UI though, only programmatically or via one of the other UIs which may support it.

Burnpost has some modest additional benefit in that it can help us deal with overpriced SBD, for which the lack of any in-built mechanism is a deficiency in the current blockchain design.