This problem is not related to the RadWindow control which is yet another IFRAME. If you put a standard IFRAME on your page with src set to the Url you are having problems with, e.g.
<iframe src="http://clients.mindbodyonline.com/ws.asp?studioid=3366&stype=-9" width="500px" height="500px"></iframe>
you will experience the same exact behavior.
All the best,
the Telerik team