Amazon is probably doing some unfair or even illegal things.

But if your business is buying something from supplier X, adding no value at all and selling it on ecommerce site Y, then it makes perfect economic sense for either of X to sell to Y without you or Y to buy from X without you.

It's in the consumers interest for that to happen. The only situation where this wouldn't happen is if volume is too low, in which case you're adding some value by aggregating demand across a few channels.