That's a good point. Amazon and Google are direct competitors in tablets (Nexus vs. Kindle), phones, app stores, cloud computing (I'm using both for Recent.io), local delivery, living room TV, product search, etc. You could make a reasonable argument that Google competes with Amazon in more categories than it does with any other company.

It's reasonable to disclose this in any case, just to remind the reader. (The WSJ does routinely.)