The point of the discrepancy between purchasing Robux and converting to fiat is to allow Roblox themselves to be able to pay for the infrastructure and support they provide to developers. If the rates were equal, there would be an effectively equal amount of money going into the platform as going out of it, which wouldn't leave enough to pay for their expenses.

This system is known as the Developer Exchange programme <https://en.help.roblox.com/hc/articles/203314100>. The differing exchange rates are a necessary part of how the platform works and should 100% be legal. Analogues to "company scrip" or the like commonly used to unjustify the system are propagated by people that don't have a coherent enough grasp on the platform's economic system.