How to publish under a Creative Commons license?
Let me build upon excellent Wrzlprmft's answer, adding a few things:


there is a de facto open access standard license, and that is CC-BY. It is similar to public domain, and I'd strongly suggest not to use CC0, as you would waive also the attribution of your work. CC0 it is used for databases and data, where CC-BY licenses are trickier.
institutional or disciplinary archives as ArXiv are heavily used in some academic communities (eg. math, high energy physics) but they do not provide peer review. It is customary within those communities to archive pre-prints. You should try to understand the customs in your very own community: it's very important if you want to make the best choice (and if you want to foster open access publishing in your discipline). 


As per your last question:
why do so few researchers seem to be concerned with these ethical problematics?

Because their priority is their academic career. Academia is built on personal, scientific reputation, and the current system (with Impact Factor and other scientometric indicators) is completely unbalanced towards old, authoritative closed-access publications that provide the "authority" a young researcher needs to gain reputation between its peers. You need to publish a lot, and in reputable journals. You will advance, get grant, get tenure on your publication (and citation) record. 

Open access publishing (as a model) is relatively young, thus it is difficult for open access publications to gain the same reputation as those other journals; thus researchers don't want to publish there, thus the old, closed-access system survives. 

Often, the rational, selfish choice of the researcher is enough for the ethical choice to be bypassed. Ignorance of alternative models (and ignorance of the intrinsic flaws of the current closed access system) do the rest.