With some it may be on purpose, with the thought that the publisher is so small, or self, or POD, that maybe mentioning where the work came out will not be a boost. But with the great majority, it's clear that the author and/or reader/reviewer just never considered mentioning who the publisher is.
And it bugs me not only because it makes my list-tabulating that much more difficult, but because there's this inherent assumption that now that the writer<-->reader connection has been made, the channels that made that connection are unimportant. As if the risk, money, support, etc of the publisher are just not important now that the deed is done.
I know darn well that's not what most authors think of their publishers, but it still surprises me when there's no mention of them.