I wouldn't hold my breath for Google to allow Roku to search its Youtube database, considering how little they care about supporting the app at all on Roku (comparing Youtube on Google TV and Android to Roku is a bad comparison for obvious reasons).
While I can see your point that Google, Inc. may favor its own devices and software platform (GoogleTV and Android) as you point out, you seem to have omitted my comparison of Roku's search with that of TiVo. TiVo's search does include YT as part of its database source.
I also agree that when it comes to Roku, Google has always been resistant to working with them. For the longest time (quite awhile back) Roku was pretty much the only popular streaming platform that did not have an official YT app. And "unofficial" versions were asked to be taken down. I remember those days and it is the reason I learned to use alternative YT players like "MyVideoBuzz".
But, who knows what future updates and enhancements will Roku search obtain. All I know is that it will seem "incomplete" until it includes Youtube, Vimeo, Rooster Teeth, MetaCafe and other video sources of free video content. Again, with so many free TV episodes, movies, etc. on YT wouldn't it be nice to catch a free episode of your favorite show found from Roku search instead of it only being available in a subscription required type app (Netflix, Hulu, etc.) or just buy episodes (Vudu, Amazon, etc.)?