I highly doubt the parent poster was in any way contemplating how to hide child pornography, that is quite an unfair interpretation.
Hiding illegal material is probably the least of concerns for most people here, but there are plenty others, I've written a few of them below.
One issue out of many, is that many who has worked with and used computers for decades has encrypted drives or volumes in a drawers, or closet which they have forgotten the password to, and could in a very theoretical sense be held in contempt if they were to be prosecuted for something and the prosecutor by some reason got a warrant for that drive.
Another is that according to what I have read the prosecutions appears to have enough to convict, so maybe setting a precedent that could be - but not necessarily is - dangerous to society might not be warranted here.
Yet another is that lots of people feel that it is their right and liberty to be able to store their personal information where it is safe from anyones eyes, even when it is completely legal. The inability to keep the private private feels like having a camera in you bedroom that you have been promised will never be turned on to film you, but the blinking red LED causes a relenting unease prompting you to wear a pyjamas to bed, even though you really like to sleep naked. This is called a chilling effect, where knowledge of surveillance or that someone can probe your most private writings and pictures causes you to not write and makes those images in the first place.
Every crime is a tradegy, but nothing creates more tradegy than legal systems or governments run amok. History teaches us that no government is safe from becoming a tyrant. This is why law enforcement sadly must always be ineffective, as the power wielded by government through law enforcement would otherwise become far too great. This is more important today than it has ever been because today we could probably implement an almost perfect police state, a perfect prison, the perfect nightmare from where there is no return. A place where no revolution is possible, no dissent is ever visible, and the fear is total and all encompassing.
The problem with privacy extremists is that they are ruining the ideal of justice with their hardline stance on keeping things secret from the authorities. In effect, this stance is just pandering to child abusers, terrorists, etc. while offering very little positive to society in return. Everyone needs some level of personal transparency to the rest of society, for the collective good of society.
To be honest, it's quite disgusting that you're most concerned with how to hide such horrendous material.