What happens if he refuses to accept that block, and another miner creates a block without that transaction, and he accepts that. Won't this create a soft fork, and it will be up to other miners to decide which fork to run with. My guess would be that most nodes and miners won't want the overhead of checking transactions, so it would need to be an automatic check in the core software. Spammer support seems to exerting an influence here, and that is a bit concerning.
A miner's refusal to include a specific transaction and another miner's creation of a block without it does not actually create a soft fork because it's kinda like a normal part of how the bitcoin network operates. A soft fork is actually a network upgrade that introduces a new stricter rule which is backward compatible and requires a majority of miners to adopt.
Actually, in the scenario you are talking about no new rules are being introduced. Instead miners are simply exercising their right to choose which transactions from the mempool to include in their block to maximize how much they get from fees. The only thing is the excluded transaction remains in the mempool waiting for a different miner to include it in a future block which is the reason why if your transaction is blocked by many pools you'll need to rebroadcast so others can see it.
Hope you get it?