Why No Support? So.... What is building past the world border going to benefit you in? The only thing you would do with the build perms past the border is if you are trying to use a glitch that benefits you.
Eh maybe put it up on the /f map or something? That wouldn't hurt. Nor would it do anything to build perms... *cough* mega *cough*.
The problem is that the world border is uneven, it is like the bottom bedrock where some parts are closer while others are 1 block farther. I would totally agree with you and I wouldn't have a problem if they just had the border walls even.
The border is a circle. Also if they showed the border by placing blocks then they would disable building past the border. No, if people mess up claims you can build a wither right next to a base.