"If you're a copywriter working through freelance agencies, those agencies are not your team-mates; they're your competitors. Read that again."
If you're handy with a keyboard and you've dipped more than a fingertip into the topsy-turvy world of freelance copywriting, I dare say you'll be familiar with the notion of the “free trial”. It might go by an alternative name, but it's the one where the hirer ever so politely explains that before you're given paid work, you either write at least one free article to “authenticate your ability”, or you take an extended hike.
The condition is rigid, and applies whether or not you already have a catalogue of content online. And the employment? There isn't any. You're “trialling” for the chance to be exactly what you already were. A self-employed freelancer who will still need to prove themselves over and over with each individual task. You're not being appointed - you're just registering. Under these circumstances, is it fair for the would-be hirer to demand a “free trial”?