If a potential customer's requirements are not met by a cloud solution, then the SAP on-premise solution is positioned (regardless of whether they are an existing SAP or not).
Keep in mind that Ariba has an European data center and new European customers can take advantage of that option. Additionally, Ariba is building out other data centers in other countries that have unique regulatory considerations.
If you would like to discuss further, then please send me an email at Jason.Brown@sap.com.
Regards,
Jason