For sure. It isn't free, they are just making good use of otherwise unused capacity. Much like spot instances can be dramatically cheaper they are using the cheapest available resources. Plus they are happy with being preempted at any time or their work being slowed so they are working with an even tighter contract then GCP Spot Instances which have minimum runtimes and preemption notices.

It is hard to decide if this is unfair or not. It is true that Google doesn't offer this exact ability to external customers but most customers wouldn't want this type of very unreliable infrastructure. If there was a market Google or Amazon would likely offer it (basically a super-spot which has less guarantees)