I do think they should be allowed a bit of privacy, as she is a minor (and a younger one at that... not like a 17 year old... I think she was actually 14 when first missing?), but police will be more reluctant to help future missing kids' parents if they know parents won't cooperate once the child is (thankfully) found.
If I had to guess, Abby either hasn't told parents who helped/kept her, or it was a condition of hers that she would return if they didn't aid police in convicting the adult. I'm sure Abby's mom is only thinking of Abby right now (which is totally understandable) and not the overall picture (Is there a dangerous adult out there preying on other kids, etc.)
EDITED to clarify that obviously the "non cooperation" is speculative at this point, I believe, or was posted by a non-credible MSM or twitter feed.