Reason why brand logotypes are in lowercase?

Amazon, eBay, Facebook, Flickr, Intel, Citibank, Macy's, BP, Vitamin Water to name a few, have their logotypes entirely in lowercase. I assume the reason is to make it more casual and friendly.

Is there an explanation from the UX perspective for this trend, and are there any studies to prove that such a trend is found to be psychologically more approachable to the users?