It's like that on the 757 as well... and IMO no it's not ok to just take that space for yourself..half of it belongs to your seat mate. But people do it ALL the time. I don't mind as much if there's enough foot space for me elsewhere. But sometimes because of equipment boxes under the seat some of my foot room is within that middle section (on my side). I've gotten accustomed to putting my foot in that space before my seatmate arrives if I want to preserve it for myself.. if someone sees you're already using your half of the middle with your foot they won't put their bag there. I've only asked someone to move their bag a handful of times in a friendly way...generally people will see that they're actually taking up some of your room they're happy to oblige.