My parents were these types of people. It's bullying. A grown adult with a degree and full-time job justifying this behavior is ridiculous. Where does a kid with a part-time minimum wage job and undeveloped brain get the money to afford a fully independent life? They simply can't and the adult saying that knows it. It's a way to feel good about exerting power.
If a person is so perturbed at the idea of providing something to their child while simultaneously not having full surveillance into its use, they should've worn a condom. People like this make stupid decisions, have a kid as a result, and then make it the kid's problem that they exist in this world.
Unfortunately, some parents don't understand the idea of respecting boundaries of their child. It is _their_ child after all. An object to do with what they wish.
I do see that you yourself do not do this thankfully. But that logic is still incredibly flawed and perverse in my opinion.