>Note that this kind of gameplay is very usual for strategy games: We have "magic borders" in Settlers, where a tower will keep you from building. Same thing in Civilization, where it's the cultural borders or the pretty arbitrary "x tiles apart" rule. Or the creep in Starcraft.

That is true, and it is also true that this is widely accepted.
But there are as many games that use the free approach (AoE, WCIII) where building a base right into the face of your opponent (if you can actually pull it off) is a very valid strategy and also widely accepted :)

>The strategic angle: If you can't build at a location, it's a position that you haven't currently claimed with your flag

The other viewpoint would be: If your enemy can build at a position your don't like (in front of your base) it is a position that you did not protect enough - since it usually takes more effort for the enemy to get there than it would take you to prevent the enemy from doing that. (It only worked in CR because the construction-material-rule was not usable for melees and therefore deactivated -> you could build hard to destroy stuff with only a conkit)