I think it's important to be very careful about assuming a parent thinks his child "can do no wrong". The parent may not make a big deal about the child's behavior in front of someone else (and with kids of SOME ages SOME "wrong" may be along the lines of, say, a two-year-old taking a toy away from another young child. (One parent may understand that if you put a two-year-old together with another young child there's the chance that will happen, so that one parent may not do more than separate the children and return the toy. Another parent who doesn't realize that may think some big, dire, consequences should have happened to the two-year-old.
So sometimes "can do no wrong" isn't just in the eye of the beholder, but in the eye of someone who doesn't understand two-year--olds very well.
That aside, though, there are parents who are not about to make a big deal out of their child's behavior in front of someone else - out of a sense of aiming to preserve the child's sense of dignity and/or aiming to protect what may (more than many people realize) be a matter of a child's not being humiliated in front of others.
There are also parents who are sure enough of themselves that they aren't going to do/say/believe something just because someone else thinks they should.
If one doesn't like what someone else's kid does stay away stay away from that other kid or graciously cut short a visit. I'm assuming it's a home/personal situation because schools/preschools deal with parents, so "some other parent" doesn't have to spin their wheels over them.
There are people who have an "all-us-adults-versus-all-everyone's-kids" mentality. There are also people who are more worried about getting peer approval (from another adult or parent) than they are putting what is appropriate and fair (and more discrete) to/for their child. (They're the ones who will hit a toddler in a store because they think someone thinks they should.)
Some parents are secure enough, and respectful enough of their child and relationship with him, that they won't betray him/that just to satisfy someone who doesn't/shouldn't matter all that much (and who may not understand one or another thing.
It's never good to presume to know what someone else thinks or doesn't think (particularly about his own child). That's a "mind-reading kind of thing"; and regardless of who thinks or understands what, people can't know what other people are thinking.