Because it is contained in Scripture, "Behold, I lay in Zion a chief cornerstone, chosen, and precious: He who believes in him will not be disappointed."
So really, the position that the Jehovah Witnesses take in not saluting the flag and consciencious subjectors, and so forth, is really not a scriptural position. We are to submit to the ordinances of man, to every ordinance, for the Lord's sake, even though we may grind at it, for the Lord's sake. It's God's will that we not be an offence. We might put to silence the ignorance of foolish men.