I think your post helped clear up for me why Matt's mail bugged me.
As you say, reputable companies often do give influential people products, and then hope for reviews of conspicuous use. But in my experience (and yours, as you explain it), these products are usually often not given _on condition_ of a review or such use.
If there is a contractual agreement to endorse a product in return for some form of compensation (a camera, or money) then yes, that should be mentioned by the company, to be ethical. (In the US, you will often see in small print "paid endorsement" on TV ads.)
If Matt had written me an email that went:
"Hi Stefan, we believe in our product so much that we're doubling your warranty from 3 to 6 years. In return, I hope you will consider joining us on Facebook, in part also so we can stay in touch. We also appreciate testimonials, which you can make <here>."
Now _that_ is an email I have no ethical problems with. I would even be motivated to go blog their gesture. Can you see the difference?