Posting photos on the internet is not the same activity as decoding and watching a signal. If you have camera that takes nude photos through clothes and you look at those photos in the privacy of your own home, there is absolutely nothing at all stopping you. I personally would not do it, nor would I condone it. But it is not at all the same thing as receiveing a televsion braodcast. Posting on the internet is akin to re-broadcasting. Also, light reflected off someone's body is a circumstance beyond that person's control. It is not the same thing as DTV's intentional, but not unavoidable, action of broadcasting.
No, it's not. We wear clothes to block the "visible" spectrum. One could make a case that failure to wear clothing that blocks IR is just as irresponsible as going naked.
My point is that the law only deals with a tiny percentage of lawbreaking. Society exists because most people follow, for the most part, the spirit of the law. Reading another person's broadcast when they have taken steps to prevent it is just a dirty, scummy thing to do. It marks a person as having an infantile sense of ethics.
Even if it were "legal" in the sense of being unenforceable.