Yes sure, but the OPs examples weren't the normal 'Fred, don't forget to say thank you for the chocolate bar'. They were an example of a child that was acting out of sorts and walked away before parent was able to remind them, and an example of a little kid not saying hello when spoken to, which as has been discussed at length, could be due to a number of reasons!
By all means, we should try and encourage politeness, remind our children to say please and thank you. But if we do this, and the child walks off we need another approach. Or if we know our child is shy etc.. and may struggle to say hi to an adult stranger, we need a longer term, more thoughtful solution.
Furthermore, if a child does not discipline their child in front of you, we shouldn't automatically assume they are a bad parent, but rather maybe more kindly, think they might just be handling the situation differently to how we would, and maybe they'll talk with the child later. Our way is not always the only or best way.
I don't judge a parent badly for these one off little errors of judgment/mistakes/instances of less than perfect politeness. My children both have a lovely group of friends with lovely parents. For the most part they are all well mannered and polite. But they aren't perfect. All of them, pretty much, have had occasions of not thanking me for a take home present after a party, or rushing out the door forgetting to thank me for having them, or chatting as they got down from the table without remembering to thank me for the meal etc... They are young and learning. The perfectly polite 8/9 year old that never slips up and never ever makes mistakes with manners is more of a concern I think.