Why can't I delete heapster and kubernetes-dashboard on gke namespace=kube-system

As mentioned in another answer, you cannot delete them directly via the Kubernetes API; however, you can delete them indirectly via the Google Container Engine API.

To remove the dashboard, run gcloud container clusters update $CLUSTER_NAME --update-addons=KubernetesDashboard=DISABLED.

To disable heapster you need to disable monitoring using gcloud container clusters update $CLUSTER_NAME --monitoring-service=none (it may actually require disabling another add-on too, I can't recall at the moment).

See https://cloud.google.com/sdk/gcloud/reference/container/clusters/update for the commands referenced above.

Heapster is configured as a cluster addon. The addon manager is going to reconcile it to it's preconfigured state if you change or delete it.

You are stuck with it.