That is boring. SO I will go make a giant spam hut in the center of the entire map and no one can destroy it. It isn't a loving freebuild. It is a trench WAR, destroying bases is a tactic to winning. If you want to make a shrine or a house, go do it on a trench freebuild, not a trench TDM. I will dig through walls whether the rules tell me to or not.
If you're just sitting there building a spam hut in the middle of the map, chances are you're going to get shot. Part of the strategy is determining where you should, and can build. You're right, it isn't a freebuild. If you want to make a house, this would still not be the place to do it.
The problem with allowing players to destroy builds is that random jerks will just run around cool (and effective) bases with their shovels over, and over again until all that's left is pock-marked ground. Destroying is faster than building and requires a deal less patience so you often lose all of the structures, resulting in an arena TDM with few to no obstacles.