This is a continuation of the post below.
I though about this in the shower, and here is the best way that I can describe why I think Paul didn't bother (or need) to give an explicit condemnation of Roman slavery.
Suppose, for the sake of argument, that there are only two types of governments available for nations, A-ism and B-ism. Let's assume that A-ism is acceptable to God while B-ism is inherently evil.
Does the bible have to condemn B-ism explicity? Not if it is easily deduced from all fundamental teachings that the principles upon which B-ism rests are evil. We are rational intelligent beings, and God is not our nanny.
Now suppose Paul, touring country A, which practices Godly A-ism, encounters Bob, a refugee and a pagan from country B, a nation governed through B-ism.
What is Paul's reaction? I suspect he would praise God that Bob escaped tyranny. Then he would present the gospel to Bob.
Suppose Bob is converted to Christianity.
What is Paul to do with Bob? Have him join an armed militia of fellow expatriates? I don't think so.
One suggestion Paul might make to Bob is to return to his country and preach the gospel, even though it will certainly mean prison and possibly death.
Does that sound implausible? To me it does not. Yet at no time did Paul have to lecture explicitly on the evils of B-ism.
The story of Onesimus is analogous. Not perfectly so, for there are two additional pressures on Onesimus. One is that he broke the law (I suppose you could say the same about Bob) and the second is that Onesimus, it would seem, also stole from Philemon.