Why do you think there are clashes between parents and non-parents over parenting issues?

I just read a thread about whether or not it’s right to tell you child there is no Santa Claus. This is at least the second time this holiday season that the issue of Santa Claus came up. In this thread that I just read (late to the party, as usual, due to my own childcare issues which preoccupy me), there came some heated debates about Santa being a lie or being fun make-believe, and whether or not it has the potential to traumatize children.

There were some words spoken by some people about parents vs. non-parents on Fluther. Parents seem to feel non-parents are critical, non-parents seem to feel their opinions are not valued as much due to being non-parents (just my summarizing here).

Why do you think there are clashes between parents and non-parents over parenting issues?