Due to the Prime Directive, I believe Starfleet would only have made first contact with the Sioux-Aliens if they were warp capable. Otherwise the very means of communicating with them would interfere with their natural development.
Either Picard or Troi stated in the TNG episode "First Contact" that they wait until a species reaches that stage in their development before introducing themselves (though only after undercover anthropological research has been conducted).
The Sioux-Aliens could only be in the very early stages of warp (no faster than 1.5), but because the asteroid belt is in their system they are de facto 'landlords', so ultimately have a substantial claim to what is there--they would however lack the means of effectively mining it.
Starfleet would respect their wishes, so if the Sioux-Aliens asked for no further dealings with them, Starfleet would respect that--though they could always change their minds in the future.
But thats just some more thoughts on the topic.