It is perfectly reasonable to think that a history of the ancient world penned mostly by Greeks or Romans would exaggerate their good points and their enemies' weak points. Since the Ancient Persian Empire
was one of the biggest and most enduring enemies, it may have received a disproportionate share of negative characterizations. For this reason, when modern Persians/Iranians write complaining of the Western portrayal, I read carefully, in part to figure out what the non-native writer of English is trying to say and in part to see whether there may not be something of importance. Two posters on the threads about 300 seemed to say that in the Battle of Thermopylae, the Greeks may have had slaves, but the Persian side (made up of various ethnic groups because it was an imperial army) definitely did not have slaves:
The producer protrayed the Persians as Barbarians and devil worshipers, ruthless warriors, they protrayed the women as prostitutes, and slaves.
Even MSNBC http://www.msnbc.msn.com/id/17599641/
Says the Persians of (300) are depicted as "decadent, sexually flamboyant and evil" but not uncivilized and not slaves.
Perhaps I don't understand the slave part of the complaint, but I read it as complaining that the 300 movie showed the Persian army with slaves. (Did I miss something?)
If that were true (that unlike the Greeks, the Persian side had no slaves), it would indeed be worth at least a note here.
So I did a search.
To avoid the obvious bias of western literature, I looked for monuments that could be attributed to the ancient Persians, specifically, Achaemenids. I found archaeological/epigraphical articles on cuneiform inscriptions which list many sales of slaves. One instance is a sale of slaves from March 24, 414 B.C. while Darius II ruled the Persian Empire.
"Fifth Century Nippur: Texts of the Murasus and from Their Surroundings," by
Matthew W. Stolper.
Journal of Cuneiform Studies, Vol. 53. (2001), pp. 83-132.
Another relevant article is
"The Neo-Babylonian Text from the Persepolis Fortification," by
Matthew W. Stolper.
Journal of Near Eastern Studies, Vol. 43, No. 4. (Oct., 1984), pp. 299-310.
Here is one of the relevant passages from Persepolis:
received from Marduk-belSunu that silver, two minas and twenty-five shekels, the
price of his slave. ("1-5) Bel-iddin assumes guaranty against suits (brought by) improper
or proper claimants (to the slave) (and against suits claiming) the status of king's
servant, free citizen, temple oblate, (or).... (for the slave).
So, although I may have misunderstood what the modern writers were posting, the Persians (as in the families of the armies that fought for the Persian king) were not unlike the Greeks in owning slaves.