How so ? They can SEE it, but they cannot do anything about it.
If the miners decide to make a given block chain, and not another one, what can the non-mining node do apart from noticing that there's no "good" block chain around and stopping to download it ?
If economic nodes stop accepting the miners blocks, then the miners cannot do anything with their coins.
This is always the same confusion. You're talking about *economic* nodes. But there are no economic nodes, there are simply economic USERS. The fact of them having a node or not, doesn't matter, does it ? The only thing that a full node can do, is INFORM its owner. But that's it.
Suppose that you are a whale, holding 100 000 BTC. You are running a full node, to watch those evil miners. Now, suppose that those evil miners have decided to increase the block reward (that is, they change the protocol so that the next halving won't happen). They keep on mining blocks with the old reward, and refuse to apply the halving. They are not doing this in a sneaky way, they simply announce that they will apply a hard fork so that halving is not going to happen any more, for this and that reason. 95% of the hash rate is behind this move.
Your node sees this, and stops. The last block it received and accepted, was the last block before the halving. The next block in the only chain around is considered invalid, because the coinbase transaction is not correct, it isn't halved, as it should be, according to your node.
Now what ? You decide to sell all of your bitcoin because you're not happy ? What do you do with your 100 000 BTC ? Where do you send your transaction ?
You're hoping for the 5% minority miners that wanted to keep the old chain ? Maybe they do. But the difficulty is now 20 times too high for them. They can only mine one block every 3 hours or so, with HALF of the reward that their competitors win on the other fork.
Are they going to keep the "true" chain alive ? With one block every 3 hours, for the next year or so ? Or are they going to put their effort in the new bitcoin ? Suppose they do. Now, bitcoin has undergone a hard fork with no more halvings. Your node is not happy about that. What are you going to do with your 100 000 BTC wallet ?