I feel like in some way there are almost two institutions running in tevinter. There's the system of pure slavery, which Fenris was a part of and which is a key part of the slave trade, and then there's a sort of psuedo-slavery that's more along the lines of servitude that Dorian talks about and Krem says his father was forced to join to keep money coming in. Admittedly, the second system isn't all that better, but a key part of that system is consent to the status itself, a time-limited contract of particular service, and the gaurantee of payment for said service. Slavery, on the other hand, is nonconsensual, usually permanent, and generally pays absolutely zero.
Now, it seems like the institutions of slavery and servitude are effectively conflated/conjoined in Tevinter, and which sort of state a person ends up in is largely dependent on who they work for and how they entered, but think about it. If the people captured and loaded into slaver ships were unabashedly bound for the same fate as the people like Krem's dad who throw their lot into the system because they have no other choice, no-one would voluntarily enter into the system, but there's clearly a difference in expectation of lifestyle at work. The person on the ship is likely going to get the Fenris treatment, but someone who voluntarily sells themselves is doing so in an attempt to earn a living and that attempt sounds like it, for better or worse, at least works on the front of earning enough money to live on. Slaves, by definition, can't really earn a living unless their master feels like paying them. The fact that the system apparently offers voluntary subjects positions in which they can expect to earn something approaching a living, especially if they land a decent, fairly prestigious family (bit of an oxymoron in Tevinter, I know, but look at House Pavus) suggests, to me anyway, that there's almost sort of a tiered system at work that is based around your citizenship to Tevinter itself and whether or not your entered into the system voluntarily.
Citizens who voluntarily sell themselves seem to expect to end up in different situations, or at least marginally better conditions, than people forced into or born within the system proper. Many of them may not like it, but there's clearly a different expectation at work here.