My wife would probably disagree with me, but I think back to what I was able and unable to do as a child, even before kindergarten, on a weekend, it was not unusual to be out the door after breakfast and as long as all of my siblings and I were home by dinner (or called), nobody panicked and no fuss was raised.
As a dad of two girls, it's always a hassle when I take them out alone because of the bathroom issue. My wife doesn't want me to go into a public restroom alone. I don't like them going into the men's room with me because men are pigs. Even if no men are in there at the time, it's always disgusting. I prefer asking a female employee to see if the ladies' room is currently uncoccupied, then going in there with my kids and announcing my presence any time the door opens ("sorry, I'm here with my daughters"). But sometimes I have to let them go alone. My wife hates it and chews me out every time, but I tell my girls how to behave, not to talk to strangers, do their business wash up and get out, where I'll be waiting for them etc. Sometimes you just do your best.
As far as the OP's story, maybe this is a mom who tells her kids, as others have said, what to do if a stranger approaches them. They might know all about Stranger Danger. Maybe she thought (right or wrong), she'd be able to see her kids while placing her order. Maybe she was afraid she wouldn't be able to navigate her kids and the food and find a table (the "ethics" of holding a table when you don't have your food notwithstanding) without spilling everything. Maybe, like a lot of people, she just got overwhelmed by trying to jam too much into one day, she got exhausted and made what is arguably a questionable choice (or questionably an arguable choice). I can have some sympathy for her because of the daddy-daughter bathroom issue. And sometimes you just don't want your kids to think every adult is trying to kidnap them and murder them, so you give them an ounce of freedom and you hope for the best.
Having said all that, one of my creepiest moments at WDW (pre-having kids of my own) - my then-GF and were on our first trip. We got off our bus and walked to the gate to the MK, and we saw a 3 year old just running away from his parents, towards the Contemporary. They were completely oblivious to it, walking in an opposite direction, looking at a map. They had one other child, noticeably older, so one of them could have been holding the hand of this 3 year old, they just didn't. We brought it to the parents' attention, and they just rolled their eyes as if it were the hundredth time it happened...not like they'd take steps to stop it from happening for the 101st time or anything...then lumbered over to get the kid, never calling his name as he just randomly walked down the sidewalk, at any time possibly veering into the street. I found that far more ***-ish than leaving your kids at a table while you get them their lunch, in a location with more security cameras than your average Vegas casino. I know, we live in a world where anything can happen. But we live in a world where we can practically wrap our kids in bubble wrap and wheel them around like Hannibal Lecter so they're always in our site and never off our leash and something bad can still happen, and we have to live in those shades of grey where we try to keep them as safe as possible but let them know we trust them and that, while there might be monsters in this world, it's not nothing but monsters.
I know that if I tried to keep my youngest with me, it would've caused a bigger scene taking him with me because he felt he was "old" enough to do what his big brother was doing. I learned fast that post-potty hand hygiene wasn't always as thorough as I would've like had I been present to supervise, so I made sure to always have Wet Wipes or hand sanitizer to make sure that I could catch those little germy hands on the way out of the restrooms 
