He isn’t disregarding the premise. He is questioning the premise. The argument that missions should have mineable rocks is based on the premise of mining being valuable as an activity. He is allowed to challenge that premise to prove the argument is valid - as a flawed premise invalidates all arguments based upon it.
If mining has value, then there is an argument to be made for mission rocks (and there are also still arguments against mission mining as well) - but if mining has no value, then mission mining is additional without value.
A rhetorical question is the counter to an argument that is begging the question, by directing attention back to the underlying premise and demonstrating an opinion that brings the premise into question.
So: what is the value in mining, that it should be extended not just to asteroids in belts, but also into mission space?
This is actually a great source of discussion.
My answer: mining is how we get minerals to make all those fund PvP ships, so clearly it is necessary. If some people like mixing up activities by mining the rocks in their mission pockets after they clear the rats, I think that is a great way to keep people engaged in sources of necessary-to-PvP resources (loot drops from ship kills + minerals for construction); but it must be balanced against resource deployments appropriate for the security status of the system where the mission is spawned.
Some of the missions that had rocks stripped were producing rocks not native to high sec, for example, or restricted to lower-security sections of high-sec. Removing these rocks helps maintain the risk vs reward structure for high sec, which I consider a healthy change.