One Thing I Don't Understand...

...is why the MLM promoters keep insisting that their particular MLM is perfectly legal, and not an illegal pyramid scheme like all those other MLMs.

Let's say they are 100% correct.

So what?

It's also 100% legal to take all your money and throw it into the sea, or to bang your head against a wall repeatedly, or to watch paint dry. Whether MLMs are legal or not is a red herring. What matters is whether they're a good investment of one's time and effort.

They aren't.