Keep in mind that the threshold is very low. We're talking about essentially living below the poverty line so that you don't have to pay taxes.
Can't be. I mean, literally, economically, it can't be set at that threshold. You posit a society where people can be compelled by law to buy a slave before they can afford to buy a place to live - and that simply wouldn't be workable, especially as it seems to be a given that the number of slaves you can be required to own actually depends on what sort of space you live in. No, it makes more sense if the threshold is set at the kind of level that I've seen a couple of other writers assume - that you can be compelled to buy a slave or two when you reach, well, John McCain's idea of poverty-level living - only one house, only one successful business.
For people with children, it means that your kids will be stuck in the public school system, which is even worse than it is now, and they'll mostly be stuck in low-paying, dead-end jobs that are barely keeping them out of slavery.
Actually, I think the real problem maintaining your income perpetually at a level which means you can't legally be required to buy a slave would be the financial insecurity - you can be living quite comfortably at that level until something goes wrong, at which point you are suddenly at risk of slavery yourself. So I think what people who were trying to combat slavery by not owning slaves would do would be to form networks of committment - the Internet would be great for this, but you could do it in a local kind of way (and I bet people did) before the Internet - in which people commit to keep each other out of slavery.
The other problem is that most slaves are not owned by individuals, but by corporations. Even in the unlikely event all of the wealthy people who could owned slaves chose not to, those slaves would end up doing farm work, or toxic waste cleanup, or medical research, or any of the other dangerous shit jobs that free people won't do. Once you're a slave, you can't get out of the system.
Sure. For most people, keeping your income at that level in order not to own slaves would be like me not eating eggs or chocolate unless they're free range eggs or fair trade chocolate: the vast majority of what's produced is still produced cruelly/unjustly, and sold to corporations who don't give a damn. But you do what you can with the resources available to you. The fact that no slave can hope to be freed and most slaves die young, worked to death, suggests a doomed society on the brink of slave revolt - a person who has literally no hope but death for themselves and for their children is a person who can afford to do anything. But given that these stories are being written in, as it were, the pre-Revolution period, about slaves who currently have some hope of living and about owners who are somewhat more personally involved than corporations, that fictional universe includes the free people who rebel against the system by refusing to own slaves. If there are none, because the threshold really has been set so impossibly low that a person may be compelled by law to buy a slave even though they live so close to the poverty threshold that the act of buying a slave may temporarily push them into poverty and - if something goes wrong - they end up a slave themselves - then the culture is doomed for revolution within a handful of years: probably none of the slaveowners we are writing about will survive, and indeed nor will most of the slaves.