Browser Comparison - Firefox 2 b1, IE7 b3, Opera 9
For Konqueror at least it's not any extra code. Maybe some developers are used to having to develop everything from scratch, but here in GNU/Linux we have these lovely things called APIs.
Any KDE application can get spellchecking in forms with no effort. It comes with the KDE libraries. That's just a sensible way to do things, and one reason that Konqueror and applications like it will always require less effect to rock - they inherit improvements made across the platform.