Ah, that's a fair point, "unlimited" was a bit strong. The last PPO my employer offered has a maximum on the order you cite. (It does have a few exceptions and situations it doesn't apply to, however.)
My own experience here is driven by having gone in for what I expected to be a $50-$100 visit (a doctor took a look at my vocal chords); it was $4,000. How that is justified, I still don't know. (Thankfully, insurance covered some of it, but for such a minor visit, it was still way more expensive than I expected.) I don't know that "well, it can't be more than $10,000" is much comfort when you're walking in for something that seems minor, such as lingering effects of a cold (this example) or an infected finger (I was unable to obtain any pricing for this, prior to the visit).
A few years later, surgery would have cost me ~$300. (But that was under an HMO. And I got a quote, in advance!)
(While my initial post specifically mentioned PPOs, as that is what I'm used to, there are also those without insurance, which doesn't have the max out-of-pocket of an insurance plan.)