This is a bit of overreach, wouldn't you say? While the Bible may indeed be somewhat sparse in providing info on the reasons for which we are to 'obey' God (or Jesus or the Holy Spirit), those reasons are present, nevertheless. We might not like the reasons we find there, and we may think they're not sophisticated enough for our modern brains, but our dislike doesn't indicate their non-existence.
On the other hand, the atheistic reaching to make a case and frame an ethical posture often is doesn't out of sheer existential perceptions of necessity, not because of any objective right or wrong. And that's more or less the focal point of this thread.