Why geocaches are discouraged under bridges

We have a number of geocaches placed under bridges in Iowa. Of the ones I approved, it's because the owner has assured me that the bridge is part of a hiking/biking trail or it is in a rural area that doesn't see much traffic. However, as a general rule, caches under bridges are discouraged because bridges could be possible terrorist targets. If you need more convincing, here's an article that points out what can happen. (Thanks to fellow reviewer Hemlock for bringing this article to my attention.)