In a theoretical ideal world, no freelancer would be able to do any free work because every client would be ashamed to ask for such a thing, or even accept it if offered.
I realize that not all clients are ideal.
However, I HAVE HAD worked with quite a few clients who simply seemed flabbergasted at any hint of free work.
I have had worked with clients who posted jobs that I replied to or invited me to jobs, and then scheduled a time to talk. And then before they could ask any questions about their project they asked me to accept a contract offer purely for consultation time.
I have had clients who did not end up hiring me for a job, but asked me to accept a brief contract just so that they could pay me for advice I provided to them in a job proposal or interview.
I think most Upwork clients are great. I think the ones who try to get free work are in the minority.