I think it also has to do with how many results there are for a given image search. I searched for "cisco logo" and "google logo" and was able to view their logos as thumbnails when not using quotation marks. I then searched for "qualcomm logo" and "cisco logo" with the marks and saw no thumbnails. "qualcomm logo" returns 600 results in the image search with the quotes, compared to 1600 for "cisco logo" with the quotes. Without the quotes "cisco logo" returns 7800 image results.

Nice feature, but I never see it even with the examples you're giving... I guess google don't serve the same content for everybody ..