First off, these companies aren't banned at all. They are free to show them.
They can't compete unless they show their movie in a French cinema, which they refuse to.
The only way to legally watch these movies like Okja is to subscribe to these companies. If Cannes gives out an award, you can't go see it in an independent cinema, you can't buy it on disc, you must subscribe to Netflix or Amazon. I completely side with Cannes on this fact. You can't celebrate film by promoting the distributor (and in this case, their subscription model).
Netflix and Amazon are going to continue their quest to destroy cinemas, and in this case, French business. If Netflix and Amazon want to start their own awards, then they can go ahead and do it. I'll make sure to stay away from it.