The reason that it is reasonable to bring up the barbarism of the US penal system when stories come out about the barbarism of the Iranian penal system in US media is that (not saying BoingBoing is doing this necessarily) stories about Iranian barbarism are often used in the western corporate media and by warmongering authoritarians in comment boards to seek to justify yet more barbarism, i.e., western sanctions and possible wars, by suggesting that Iranian barbarism is somehow uniquely evil in a manner that makes them a threat to the world.
None of this is to say that chopping someone's fingers off for theft is not barbaric, although overall, I think when one looks at the scale of US imprisonment and practices of US penal policy, it is arguable whether it is more or less barbaric than many standard US practices, particularly the widespread use of solitary confinement, which is widely recognized elsewhere in the world as a form of torture.
Anyway, in the interest of avoiding war, which is the greatest barbarism, I often feel compelled to point out that Iranian barbarism is not unique to the world and that those that wish to oppose penal barbarism should do well to look first to their own countries and after that to others that they are most likely to influence, as opposed to countries like Iran, where it will be easy to dismiss western concerns as part of an external plot and therefore may even serve to undermine the work of Iranian reformers and/or revolutionaries.
(P.S. I hate this new commenting system)
 ....and why is it, exactly, even if we take into account that Iranian penal barbarisms that are highlighted are culturally different from Western penal barbarisms (such as solitary confinement) and therefore more likely to shock the Western reader, that we read constantly of Iran's barbarisms but we so rarely read of Saudi Arabia's penal barbarisms, where people get stoned to death, beheaded, etc. for such crimes as witchcraft? In fact thieves in Saudi Arabia get their whole hand cut off, not just some fingers, and they are not given any drugs to dull the pain. So why the difference in coverage? Oh yeah, because Saudi Arabia follows US dictates on foreign policy and oil economics and Iran does not.