The university has the right to ask you to submit your work however they want as a condition to course completion. If you don't want to submit your work to turnitin you don't have to complete the course. Nobody's forcing you to.
If the extent of your objection is that turnitin robs you of potential financial benefit of selling your work, then that's by design: that's exactly what the educational institution wants to do! It doesn't matter that selling your work to a paper mill is legal. It's against the interests of the institution, so they're not going to let you go through their programme and keep that right.