As a parent of two boys, i feel that much of the annoyance of no-children-having people is due to parents not putting boundaries for children in place.

Children scream because of attention. It means they are not getting it.

Start walking around with the kid to calm it down. Its your job as a parent. You cannot stay seated and act like "what are you gonna do? They are children ! They scream!"

No. You are a bad parent for letting them just scream.

Having said that, babies sometimes just scream without reason. Perhaps, and I mean this, if you have a baby that is prone to doing this, do not travel in confined spaces, or dine at restaurants until that phase of screaming is over.