Yes but that would reduce the SBD supply not help put it into circulation, so less useful than the method I'm using here. It would also burn far less because each SBD is worth over $3 (recently over $10) while the beneficiary only receives about $1 worth of STEEM in its place.
It is also somewhat debatable whether just burning is as effective in increasing the market price as buying and burning. There is a degree of trust required that I will burn the rewards but on the other hand it is part of a series. If I ever don't burn the rewards people can stop supporting these posts, so the amount that could be stolen this way is limited.