I'm no expert so bear with me.

My understanding is that the danger of miner control is not "double-spending" but making multiple spends and being able to pick the most favorable one after the fact.

As a non-controller if you try to double-spend, you can't know which one of your attempted transactions will be recorded in the ledger and which will be rejected. As a 51% controlling miner, you can choose the 'correct' transaction.

This allows the 51% controlling miner to engage in futures arbitrage.