There's 2 scenerios.
First, is the outpost owners build the walls and put miners in.
Second, is non-owner players sneak on, build walls and plant a miner.
Scenerio 2 is fail. During peak time, the owners WILL notice that someone walled off their epi field, and they will poke one or more holes in it, making it worthless for riveler defense. If its a one-time op during the off-peak time, building walls is time they could have spent mining and adding the likelyhood they'll be spotted and killed before they even start. It would be cheaper and less time consuming to put an armored TP in your cargo, and fit yourself to live the few minutes it takes to deploy.
In scenerio 1, the riveler owner already has the support of the island alliance, complete with Island intel/probes/walls. Getting a force anywhere close to miners is going to be so rare, that it's not worth the additional effort of walling in the miner.
Preventing ninja miners from depleting your resources is about Island defense. Roaming an island to gank miners shouldn't be the primary PVP of a game (Unless the game is named MinerGank), it has its place, but it should be the exception and not the norm.
The beta islands have been empty for so long, that many people are forgetting what it was like when they were occupied (or they joined the game with it so). Going to an Island that is being actively used is going to be dangerous, and using tricks like walls (to attempt to use the island) simply won't work for visitors.