I reviewed a paper but was not informed of the decision -- is this common?

While most the journals that I have dealt with inform me of the editors' decision, I have found that a significant minority do not.

I've found this to be the case in fields with significantly different publication practices (e.g., computer science, biology), so I don't think it's field dependent. Likewise, there seems to be little consistency in which journals allow me to see other reviewers' comments and which do not.

My guess is that hiding review or decision information is some sort of attempt to increase editorial independence, e.g., allowing an editor to more freely decrease the weight given to an unfair Evil Reviewer #3.

I really do not like it, however, since seeing the editors' decision and the other reviews helps me simultaneously evaluate the quality of the journal and editor, which are also quite important to know (and sometimes give feedback on).

For the journals I review for, I have the impression that it is more common to not inform the referee about the decision rather than to do so, except if there is some communication from the authors to the referee. Of course, this might be highly field-dependent, so without much more information, this question is likely hard to draw specific answers.

For the first question, it depends on the journal, the paper and the editor.

For the second question, when receiving a revised paper, the editor/associate-editor may sometimes read it by himself and take a decision directly without asking the reviewers or remove one reviewer. This is done sometimes if the revisions are minor like fixing some typos. Or the editor may have other reasons to do so. An editor may also ask some new reviewer to get some different perspective. In any case, it can happen.