The behavior you are observing is due to the fact that validation is triggered when element looses focus. In the described scenario, as the checkbox is focused with the keyboard, the blur is triggered before the click, executing the validation. At this point the checkbox state is not valid, thus the validation message is displayed.
However, we have enhanced the validator behavior to trigger validation on checkbox click. This improvement will be included in the next release of KendoUI.
the Telerik team
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