I'm not familiar with the new stipulations in GPLv3 and their potential effect on this, but if I had to guess, I would guess that Kendo is relying on the GPL's infectiousness; there is a clause that requires all linked code to use the same license. I suppose Kendo may make a claim or intimate that the terms of the GPL require at least your JavaScript to be identically licensed. GPLv3 may have strengthened or clarified these provisions in light of the massive proliferation of internet applications; again, I am not versed in the modifications of GPLv3 (and I'm not a lawyer, either).

They may also just be falling back on the old corporate dogma that employees are not allowed to touch a GPL library with a ten-foot pole. Or the old corporate dogma that everything needs a support contract so that managers can CYA.