You're wrong in that temp contracts have different sorts of terms. Full term temps can usually only work with one hiring customer but that is not the only model. By taking a cut of the salary as a fee, websites like freelancer and upwork are temp agencies.

Also, upwork is a shithole. You should really not use them. I spent 2 months trying to set up an account there as a freelancer and they were complete dicks for 2 months straight. They refuse to adhere to the constitutional laws of the united states where they are based, and their support department is mockingly hostile to their freelancers.

I made a broad generalization in my first comment about all of those websites being the same. Obviously I haven't used every single one, there are hundreds of them. But the ones I have used have all been the same. It's obvious you want to use them, I strongly encourage you to stop using Upwork and look for one that treats it's contractors better.