You are correct. This behavior is by design for many different reasons which I will not go into here.
The only way of achieving your desired functionality would be to bind RadDataFilter to RadDomainDataSource.DataView as shown in the online examples. Then, if you add a FilterDescriptor to RadDomainDataSource.FIlterDescriptors, RadDataFilter should pick it up automatically. That is because RadDomainDataSource.DataVIew is a DataItemCollection instead of a QueryableCollcetionView and they behave differently.
Currently, this is what our controls offer. It is up to you to decide whether they suit your concrete scenario or there is a better third-party tool for the purpose.
Thank you for your understanding.
the Telerik team