I think that's because Opera Mobile automatically creates a submit button for forms with no buttons. Unfortunately, not all browsers have such feature, and I'm not sure whether that's available for Opera Mobile on Android as well. We don't need the other buttons. It's a lot easier to just type the bbcode when you're on mobile (especially on non-touch devices).
I browse the forums on my PC, on a Nokia N86 8MP (Opera Mobile), and sometimes on an android tab (Dolphin Browser). Dolphin browser doesn't make buttons for forms with no submit button, so it's really hard to reply on threads with it.