How have they reacted negatively?

We respect the decision of the judges but we do not understand how the decision of the first trial could be so radically overturned,

I completely agree. It's hard to understand and it's hard to believe that such a serious, important verdict can switch like that.

It can really make you feel bitter about the judicial system, and skeptical about it's ability to reliably root out the truth, so I think Kercher's family is doing great with being able to say this:

"We still trust the Italian judicial system and hope that the truth will eventually emerge."

Or was there something else?

(By the way, I couldn't understand how the verdict was guilty to begin with. But from the point of view of the parents, not only would it have been too much to ask them to look rationally and independently at the evidence with respect to Knox's guilt to determine on their own there likely wasn't enough evidence -- grieving parents may be dependent on the judicial system for that -- they would have been deeply hurt from anything the accused did to hinder the case. I expect the parents would have seen their evasions as much more malicious than we would. If Amanda was a friend how could she have not told the complete truth from the beginning? Etc.)