Thanks for the report, Michael.  This is currently WAI.

Cast functionality itself is actually a component extension and so built-in to the browser by default.  By default all component extensions are opted-in to Incognito (a similar example is PDF viewer).

You can manually disable it for Incognito by running Chrome with the --show-component-extension-options flag and then unchecking Incognito support, though obviously this method is a bit cumbersome.

We don't believe this creates a privacy issue because (a) we aren't sending any information to Google; (b) Casting requires explicit user action in order to start (you have to choose to cast and select a device, all of which is done in the context of a particular tab, so you have direct visibility into what you plan to Cast).  For incognito cases we do obscure the name of the tab/URL you're casting to other devices on the network though.  

Hope this helps; LMK if you have further questions.