HI Jasua;
SAP has designed the applications like that.
Any standard analytical application will make a call for odata service to HANA XS engine which eventually fetched the data using Virtual data models. It provides the required data at a much faster speed than if the call would have gone through SAP NetWeaver Gateway system.
So I guess to make it faster SAP is using this design.
However if you want to develop some application based on query in BI/BW, you have a provision to expose this as Odata service.
Regards,
Sarbjeet Singh