But I find it interesting that they do this even though they technically can't know if the intermediate address was really under your control. Wouldn't this make any transaction out of Coinbase inherently risky? Like, if I were a troll that wanted to get as many Coinbase accounts banned as possible, could I simply set up a service that accepts payment at a specific bitcoin address, then immediately pass on all payments to a blocked address - and have everyone who uses my service auto-banned from Coinbase?

It makes sense though that tracing the transaction chains can be a reliable way to find repeat offenders. If your transaction history shows a consistent pattern of "your account -> some random intermediate addresses -> blocked address" then there is reason for suspicion. No one has that much bad luck.