Now, this is just my take on it:

Often when a site villainizes an item like a preservative there is a
tendency to cite false references or old ones, and often to put up part
of what was found rather than the full picture.

In relation to at least one of the preservatives recently being again
attacked on non-expert websites the most recent references I could find
cited on two of the those websites was almost 20 years old.  You can
find more recent things or just as recent things by searching the
preservative name and DVM in the FML archives (but read the signature
lines to see who is a DVM and who isn't).  I still have to look at the
FHL archives so you will want to do that yourself.

Remember why preservatives are used.  They stop growths which are by
themselves dangerous (infection, organ damage, death if severe enough,
and some can set the stage for malignancies).  By stopping growths they
also prevent the dangerous products some growths create (poisoning,
organ damage, induced insanity from some of the toxins, death if severe
enough, and some of the toxins set the stage for malignancies).

Have some preservatives been associated with increased rates of some
specific malignancies in NON-ferret animals when given in ***huge***
amounts?  Yes.  Have at least some of the SAME preservatives been
associated with reduced (yes, reduced) rates of other malignancies in
those same studies and others?  Yes.  Do the preservatives themselves
help prevent malignancies through their preservation effect in normal
use amounts?  Yes.

Is it always safe to generalize from rodent studies to ferret health?
No, not by a long shot.

Have any of the preservatives talked about on various sites recently been
formally studied in ferrets?  Try looking up the compound names along
with the word ferret or the word mustela in places like Pubmed.

The closest you will find is a study on carotinoids (You know, the
compound in carrots) so it was probably just mentioned as the
preservative but i don't have the article itself to know how it came up.

Methods Enzymol. 1993;214:102-16.
Related Articles, Links

Analysis of carotenoids in human and animal tissues.

Schmitz HH, Poor CL, Gugger ET, Erdman JW Jr.

Department of Food Science, North Carolina State University,
Raleigh 27695.

PMID: 8469135 [PubMed - indexed for MEDLINE]

(the only study involving ferrets and BHT, Butylated Hydroxytoluene
found)

None for ferrets and BHA, Butylated Hydroxyanisole.

None for ferrets and ethoxyquin.

None for Propyl Gallate and ferret.

Such is life...

Shall we hear strains of "There Is No Black or White, Only Shades of
Grey" now, please?

-- Sukie (not a vet, and not speaking for any of the below in my
private posts)
Recommended health resources to help ferrets and the people who love
them:
Ferret Health List
http://www.smartgroups.com/groups/ferrethealth
FHL Archives
http://ferrethealth.org/archive/
AFIP Ferret Pathology
http://www.afip.org/ferrets/index.html
Miamiferrets
http://www.miamiferret.org/fhc/
International Ferret Congress Critical References
http://www.ferretcongress.org
[Posted in FML issue 5287]