User blog comment:B214/Volume 21 info & spoilers/@comment-50.139.232.230-20160428163740/@comment-93.105.246.116-20160507155528

I think a more simple explanation is because Regulus is not exactly Sairaorg's sacred gear. The Sacred gear is actually master-less and Sairaorg tamed it and turned it into a servant. But it's not a part of him like the Booster Gear is a part of Issei or Divine Dividing is a part of Vali. So it stands to reason the Breakdown the Beast mode would cause no harm to him as they aren't exactly sharing a body. And obviously this state cannot pose any threat to Regulus itself as that would be absurd.