I agree with you, but there's also another level that I saw mentioned here, namely that, back then, nobody seemed to mind it. Today, at least some people do.
Yeah, well it's also not true. Many people in Rome wrote against the barbarity of the games, or even slavery, the most famous being Seneca:
> I am glad to learn, through those who come from you, that you live on friendly terms with your slaves. This befits a sensible and well-educated man like yourself. "They are slaves," people declare. Nay, rather they are men. [1]
They are men. Nothing more to say.
[1] https://en.wikisource.org/wiki/Moral_letters_to_Lucilius/Let...
I figure it's a simple case of what humanity considered to be wrong and right have changed. Our advancements in technology and society have changed what is required just to live; have given us room to re-evaluate the status quo.
My point is, the article in reference, seems to be amazed that people could own slaves and live with themselves; but we do much worse than own slaves, and live with ourselves pretty well, so maybe we should start wondering about ourselves first.