Fake knight of Justice alert. She's fine with buying slaves, so she probably doesn't see slaves as people. That's a nice surprise in a main side character, but disappointing that she calls herself a justice warrior.
And considering Japan only enslaved women back in their early 1920's to 1940's imperial period (and mostly the their army at that), they probably wouldn't have a problem with slavery either. And the sex slave harems that are popular over there is understandable in that women are still treated as second class citizens over there. And that the people who read them are fine with it because they can self insert themselves into sex slave harems. Because, god forbid, you have a realistic girlfriend with personality. You just want a pet slave that you can sex, and that listens to your order like an obedient slave with, while trying to "justify" it by "saving them" or having them unrealistic fall in love with their slavery, instead of hating their guts like real people.
All in all, its a young japanese otaku NEET's fantasy to have a sex slave harem (or slave harem in general) because a real relationships with real girls (or boys or w/e gender identification) is a pain in the ass (and I don't blame them because they are).
Personally I hate harems in general, especially slave harems and slavery in general, so I just avoid them over all. But I do agree that Japanese NEET authors treat slavery lightly, since its well, not a huge part of their culture like it is in the West. I mean they had slaves (or slave equivalents) in their culture, but its not treated the same over there.