That's certainly exploitation, agreed. I'm not sure why anyone would use "no other options" to argue something
isn't when that's almost a requirement for exploitation.
That said, the desert metaphor also gets us to the usual issue: arresting the water-seller without doing anything else still makes life much worse for the hikers. I don't accept the libertarian argument that exploitation can't be immoral if it's mutually beneficial, but it seems accurate to say that the exploitation is usually not the most immoral thing happening. It's a more active harm, but having one unpleasant option is still better than having zero options.
Coming back to the article: earning four Euro per hour to live in Paris is obviously unsustainable. Taking half of someone's already-low wages to (indirectly) provide them with work papers is a callous thing to do, and knowingly designing a company to provide plausible deniability for that practice is alarming. And yet... if someone is living in an abandoned car with no support and no right to work legally, earning 17 Euro a day is still better than earning nothing at all. Any reaction that claims to be about protecting the worker needs to start with those problems before it cuts off their job.