States Rights? By the Tenth Amendment, states retained rights (actually 'powers' rather than 'rights') that were NOT specified or delegated away in the Constitution.
Clearly the return of runaway slaves was covered in the Constitution, so violation of the Constitution by not returning runaway slaves does not fall under the Tenth Amendment and thus is not an example of States Rights.
I think the section you're referring to is this one. Is that right?
"No Person held to Service or Labour in one State, under the Laws thereof, escaping into another, shall, in Consequence of any Law or Regulation therein, be discharged from such Service or Labour, but shall be delivered up on Claim of the Party to whom such Service or Labour may be due."
...so violation of the Constitution by not returning runaway slaves does not fall under the Tenth Amendment and thus is not an example of States Rights.
Leaving aside the moral aspects of this question for a moment [or the fact that slavery is not explicitly cited in the Constitution], there is plenty of room for interpretation of what it means to 'deliver up', what a 'claim of the party' is and how it is determined whether 'service or labour' is in fact due. However, let's be clear that the North had no obligation to be proactive about rounding up escaped slaves, or enforcement of any other discriminatory practice the South was engaged in whatsoever. In fact, it would seem to me that States would have wide latitude in how they met their constitutional obligations in this regard, and incumbent on the citizens of other States to pay attention to and obey State laws in that regard. -btw There are plenty of accounts of Southern slave-hunters simply grabbing any Negroes they found up North and claiming they were escaped slaves, whether they were or not and whether they [the slave hunters] represented the 'claimant' or not. Wouldn't you agree?