Yeah, I did a crappy job there--mostly, I gutted a post because I felt it was unnecessarily provocative and ended up leaving something pretty mediocre.
What do you mean by "pretty uncommon," how long is "some time ago," and what are the number of reasons? Congress didn't agree that it was uncommon in 2003 when it passed the Prison Rape Elimination Act, and the results of the latest DOJ survey say that about 4.5% of prison inmates had been sexually assaulted in the last year (though admittedly about half that for male inmate-on-inmate rape). And there are obvious sampling issues that would make it fair to assume the DOJ's estimates are low.
Inmate advocacy groups will tell you prison rape endemic; prison officials and at least one study will tell you it's a media-concocted myth; I suspect the truth is somewhere in the middle.
What I meant to address was the persistent invoking of the notion that prison rape is a persistent and common form of "frontier justice" meted out against the villainous even by prison standards.
You raise a good point that I didn't include any standard by which to claim it is common or uncommon. Obviously, sexual assault is more common in prisons than out. And yeah, I consider it a problem whereas some do not. But what I intended--and I agree, failed--to get across was both the trends and the larger social dynamics. By "some time ago" I meant since when it was discovered that prisoners might have human rights, or, at least, civil rights, and courts became more willing to hear grievances brought by prisoners (20-30 years ago? When was Farmer v. Brennan? I'm fuzzy...). Also, there has been a significant trend in the numbers (flawed as they probably are, as you point out) suggesting that prison sexual assault has gone down among male inmates while it has increased among female inmates; this would be relevant to what I was thinking about in that it is generally males who invoke the whole "Have fun in pound me in the ass prison!" thing.
While I admit I did a poor job expressing it, what I wanted to convey that commonplace discretionary punishment of male inmates through disciplinary ass rape has declined, if it ever even existed. Prison rape is almost always* about the predators preying upon the weak and susceptible and not about justice--just like everywhere else that sexual assault occurs, as often as not against people whom few would claim deserve it.
Frankly, I think it's much more likely that Sandusky gets beaten to death with a broom handle.
*I have heard and seen credible anecdotal accounts of punitive prison rape, but such incidences seem to be the exception and not the norm.