So who cares if a non-mining node rejects a block?
If a miner is mining blocks that the thousands of non-mining nodes are going to reject, then they simply aren't mining, they're just wasting millions of dollars of energy and hardware for the privilege of making blocks that nobody recognizes. Basically, they're mining fool's gold, except they don't even get to keep it.
What does it mean for a non-mining node to reject a block? Does it mean they refuse to mine on it? Because it's not like they were going to mine on it anyway.