Ponzi schemes are illegal. MLMs are legalized Ponzi schemes. The outcomes are similar but one has a veneer of legitimacy that makes it more dangerous for some people.
MLMs are pyramid schemes, not ponzis. For example bitcoin is a (somewhat decentralized, but centralized among major mining companies) pyramid scheme while Tether is a Ponzi.
Both pyramid schemes and Ponzis are illegal. MLMs are illegal in many places. In the US they were successfully able to lobby their way into carving out a legal niche in the 80s. They are also the reason supplement safety is so loosely regulated in the US.