Don't agree. To me leaving a stroller open and putting a child on a seat is no different from taking up two seats for yourself. Sure, you can do it. Doesn't mean it is the right or nice thing to do.
Also, I have seen, both on the buses and the monorails, people pick up their children and hold them on their laps to make more room. Which I think is very nice of them.
I agree that it's absolutely nice when people put their kids on their laps and try to take up as little room as possible. Still, you never know what someone else's circumstances are.
It's possible that someone's collapsed stroller plus all the stuff they have in and around it could take up essentially the same amount of space as the unfolded stroller and so folding it wouldn't make a difference. It could be that they don't have enough hands to hold onto the folded stroller, all of the belongings, and possibly their kids without some of the stuff falling into other people. There might be one parent sitting and holding onto kids (no free hands) while the other parent is standing and doesn't trust their balance enough to hold onto a folded stroller and remain upright as the monorail moves and jostles you around. For parents who sit with their kids next to them and leave their stroller empty and unfolded, maybe they are worried about the kid's safety in the stroller on a moving vehicle when other guests lean on it or don't want others butts in their kids face, so they take the kid out. Maybe something got spilled in the stroller so they had to take the kid out. Maybe the kid isn't sitting on their lap because the kid is too heavy or the parent has leg problems that would cause them pain if the kid sits on their lap. Maybe the kid has a disability that makes it difficult for them to be touched or close to others and the parents are doing their best to give the kid their own seat and a little buffer zone by sitting next to them and having the stroller in front so their kid doesn't panic. Maybe its easier to have the kid sit in their own seat than to try to balance them on their lap for whatever reason. Heck, maybe the toddler got a dirty diaper just as they were boarding the monorail (or a kid had puked all over themself and though the parents tried their best to clean it up, the smell still lingered) and the parents were trying to keep the kid as far away from everyone else to be respectful of them so as not to make other passengers smell it. Maybe the parent isn't feeling well and they are dizzy or nauseous and it would cause them discomfort to physically hold their child on their lap and they are trying their best to hold everything together to get their family back to their resort.
Sure there are some people who don't have any particular reason, or who could fold their stroller or keep the kid inside it or on their lap, just like there are people on the buses who could give up their seat and stand or put their kid on their lap instead of letting them have their own seat.
But you never know what someone's circumstances are or what they are dealing with at a particular time. There could be very valid reasons for the way they are doing things.
If people are able to make changes to take up less space then of course, I agree that is the right thing to do, but I'm not going to judge them when they don't, because I don't know what they might be dealing with. Again, everyone is responsible for their own self and it's up to them to make sure they have room/ a seat/ whatever or they can wait for the next one.