This is stupid to say the management node is free, you're just paying for it in another way. Now if they said they were able to run the node so cost effectively that it's just included in the hourly node cost, then that's a plus.
Whether it is absolutely cheaper than GCE or whatever, and if master management is a requirement, is a separate comparison based on the solution. Cutting out the management node costs removes a barrier to entry and creates a more streamlined product.
Oh, it also makes their Kubernetes services usable within the free credits they give out on Azure via their MSDN subscriptions... so suddenly millions of devs can use AKS for demos and temp-hosting "for free", which is huge in creating mindshare in the Enterprise...