Probably because of this - if you render a region and that region now has a new bright part in the image close to the edge, the bloom and glare from that region would have to spill out into the rest of the area outside of the render region. (Or if a previous bright point was now not so bright, or if the color had changed). In other words, what is IN the region could affect Bloom and Glare outside of the region.
Since Bloom and Glare doesn't take that long to calculate, best thing would be to have it dumped, and recalculated for the whole image at the end not just within the region (is this not what is happening btw, are you saying that the new recalculated bloom and glare only appears inside the region, the rest of the image outside of the region has no bloom and glare?)