The mostly likely 'reason' is that the vast majority of porn s probably captured and snippets (who has time to watch the entire plot of the production) and tends to be a violation of copyright. If you cannot prove that you own the copyright , then you can probably be charged with that 'crime' until it is proven that you do have a legit copy, Even if you have the DVD with you, that fact that it is one your computer probably means you violated some standard or assumed to be present licensing agreement. So unless you starred in the production yourself, they will probably use that angle to charge you with something.
It is interesting and at least a sad commentary how there is such a disproportionate concern. Here some military site is shown in the open and people complain, even though (as another pointed out) a rail line and highway run right along the fence line. As a conspiracy theory, anyone have a guess what secret, strange flash can be seen in the middle of the base (@ 56 3'16.70"N 4 49'5.95"W)? Look at the elevation, there are plenty of surrounding hills from which people can take pictures, such as at least two on the Panoramio layer on Google Earth; http://www.panoramio.com/photo/1100160 or http://www.panoramio.com/photo/1100421
This was covered just last week on the Oogle Earth site [http://www.ogleearth.com/2009/03/sun_stupidity_w.html] There is a small, old dam in rural south Carolina that is quite obviously to have intentionally been pixelated [35 1'13.34"N 81 0'27.92"W], though there are plenty of user photos that show the thing and one can even use the new GE 5 feature to see old pics that clearly provide details.
As others pointed out, the only folks with an extreme interest in such a place and the means to do anything of note would already have their own eyes in the sky that can gather much more detailed data. More security theater, IMHO.
It also provides closure for other cases. What if a lifetime sentence was really that long. If someone was accused of multiple crimes that carried that sentence and you are a victim (or family member of the victim) would you necessarily want the prosecution to stop after the first case and go "we got all we can get, why even bother proving the other cases anyway?"