So who cares if a non-mining node rejects a block?
Because no other node (including other miners) on the network will then accept any other blocks built upon that bad block and consequently no other transactions built on that block will be accepted. So no miner will be able to spend the coinbase transactions - which means no reward for the miner. Which means they aren't gaining anything, but are losing the entire block reward spending their electricity for nothing. And wasting the capital invested in mining hardware.
Are we on the same page that the only node's vote that matters is miners? Because the only vote you have is whether to mine on a block or not. This act of mining on a block signifies you accept it. So what do non-mining nodes have to do with this process?