Seeing is indeed not thinking, but imagine you lived in a glass house. Every action you took open to the world to see. It would be a pretty humiliating existence. Sure, the parallels are not exact as there would be more than one viewer here, but with mental probing it is magnified in that any event can be put under scrutiny. Every moment that is seen can be studied. The difference between this and a fingerprint is that one is a much richer and more voluminous collection of data. A print is like a 1kb computer file that says "yup, this is who you think it is". A summation of all of their senses is like every other file, image, video, audio, text. The scale of it sets it apart.
I would argue that it really is not that hard to make your senses lie to you, since some of them literally operate by tricking your brain. You might not have voluntary control, at least not in our world, but you certainly could if we invoke magic. The very means that allow mental probing to be a thing can work to shield a mind or negate the veracity of the information gained.
As for lie detecting spells, as they do not invade the person's mind and have free reign across everything that person has ever experienced, no I am not nearly as opposed to them. I can't say I am a huge fan of overriding a person's free will, but when the stakes are high it is a relatively acceptable option. Certainly when compared to invasive procedures that are not consented to.
See, I disagree with this. I should note that I am extraordinarily opposed to actual mindrape and mental invasion - if you want to use an example of that, look at what we did with Vivi. It was for a good end, and it might have had good results, but it was still an unlawful act (we'll leave morality aside - you could spend an entire week debating the intricacies of it). I do not feel the same way about taking someone's sensory registry. To me, that is much closer kin to, say, taking a few locks of bloodstained hair off someone to see if the blood matches up with that of a victim, or once again, prints. Your senses aren't who you are, but rather what you did. They're action data, and not thoughts. Scale is not an argument I can respect, because it doesn't really excuse committing a crime. You did it, and now some of your rights are briefly voided. You can't walk away, your possessions aren't yours, and neither should your senses be yours if their information can help society as a whole. In contrast, someone who is innocent and not a suspect would have to give his or her consent before going through a sensory scan, same way you don't jail someone who is a witness. If they refuse, they refuse. If not, you get the information with as few distortions as possible.
Now, you can try to argue that your sensory registry can be warped by your brain, and thus becomes a memory. This, to me, is a much weightier argument, but it has some flaws. Mostly, the flaws I can see lie on defining what, exactly, counts as a memory. Is it anything stored in your brain? Is it something with emotional attachment? Or maybe something else? This probably has to be defined to continue the debate any further. Personally, I err on the side of thinking that a memory is your complete, multisensory recollection of an event, as interpreted by your brain. The bits and pieces of it are the building blocks of it, but not the memory itself.
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Twostuck, Homestuck RPG: http://www.gamefaqs.com/boards/8-gamefaqs-contests/63360266