I think there have been lot of discussions about this in the past. The journals need to support their site and pay staff, thus they need to charge. However, the amount of fee some charge e.g BMC and Springer for open access is quite exorbitant.
Some journals are funded by organizations and governments, hence they don't charge APC. Nonetheless, those charging APC cannot be blamed because they have to settle fees for website maintenance and editorial staff even though the production is paperless nowadays.
The "dark side of the moon" is that an indipendent researcher could not have the possibility (if he is no.....rich....) to publish . A lot of article published on Springer for example or Hundawii et al.....are payed by academic institutions or pharmaceutical groups .
I assume the main issue here is about open access (OA) publishing, since closed access are free but come with an embargo period. From my perspective, there is no such thing as "unethical" when it comes to charging for publication fee by OA journals. The authors have the option to opt for subscription based journals that have 12-24 months embargo periods. The mere fact that we do not agree with the charges imposed by OA journals does not make it unethical!
Mind you the publication fees of OA journals are merely fractions compared to the total funds spent on the research. There is no reason why a research that expended $20,000-$30,000 or more will not pay $1,500 for OA publication of its findings.
On the other hand there are ethics related to publication as stated by the committee on publication ethics (COPE): http://publicationethics.org/. This is another great perspective on publication ethics: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1804120/. However, these are core principles relating to publication from MS preparation to publication.
Your reasoning is correct. Probably I do not clearly express my point of view .
That is : how does an indipendent researcher or a small indipendent research group to pay expensive fees claimed by certain scientific journal ?
And again : the conflicts of interest concerning the examples you propose, appear to me (and not only.....) difficult to me to reconcile with "ethical" documents proposed by COPE. That is.