The Amarr Epire is vast and there's space for all kind of forms of slavery there. The PF isn't consistent in how it treats the issue of slavery and those inconsistencies can in part be resolved by this. Some inconsistencies might be something that is a real inconsistency in the Amarr cultural sphere.
And example there might be the view of slaves as subhuman (which isn't by the way not something held by all Amarr. In fact, it was first brough up by players and CCP latched onto that and now with Source has been made into the majority view) and the justification for slavery which is education and spiritual enlightenment. The idea of subhumans, if taken literally of course, is quite directly opposed to the idea that people can be educated and through education (and penal labour) be raised up to a higher status of full citizenship becoming pretty much 'Amarr'.
A lot of the fiction on slavery seems to me - from my European perspective - to be drawing on the idea of slavery in the US, plantations and the like. Even in the case of bureaucratic slaves and such it always does stress that the slaves do in some way suffer from that state consciously. There are societies where certainslaves - like in the bureaucracy - had openly a higher status than some freeman and whose positions were seen as desirable, who were also better off then most commoners, at least materially, and arguably even had more freedom, depending on the definition one gives for that.
I think the valuable thing about Amarr slavery is as something by which we can look critically at our own societys, where they grant freedom and what kind of freedom (formal versus actual comes to mind there, primarily) rather than using it as something that we can 'enjoy' from a perspective of feeling like it's something we've risen above and being in the past.
But that's just how I prefer to treat it, so whatever floats the boat, I guess.