Although I see your point you are trying to make, I want to mention that moving the validation to the viewModels (which is usually a good idea) or the data source would be counter-productive in the case of Kendo. The reason being is that many developers who use Kendo UI employ another method for managing their models other than the Kendo Datasource and Kendo MVVM. If all the validation was moved away from the opt-in validation Kendo provides into the MVVM or Datasource this would be tying developers into doing something many would prefer to do otherwise. This is highlighted by many of the project currently under development in labs.kendoui.com
I believe that if your are REALLY that concerned about how clean your data is, I suspect you should be putting Validation at each point along the way by way of keeping validation off the transport layer (which is one of the things the Datasource manages). Put validation on the server side on your 'real' models if you need that level of validation.
Finally, I just wanted to mention that another very large software company 'forced' us into their 'validation on the models' in ASP.NET web-forms which (shockingly) really irritated a lot of developers by forcing them to do things and tying their hands from doing really neat 'other' things.
Just my $.02, take it or leave it - I just wanted to put some food for thought on the table.