Your explanation is too simplistic. I've found magazine subscription pages where the link to the pdf is display:none in css. (I downloaded their entire back catalog.) This isn't that they missed a few routes to files when securing things, but that they are utterly clueless. Invariably, such software projects employ a number of contractors who for whatever reason can barely cobble together the functionality that is repeatedly demanded by the clients, let alone any of the common-sense features that these people fail to realize that they must also nag for.