The term "reasonable expectation of privacy" has a very specific meaning. It is a test that courts use to determine whether a search violates the fourth amendment[https://en.wikipedia.org/wiki/Expectation_of_privacy].

If you choose to start shouting at someone on the street and the police listen in (without a warrant), then you can't claim that your fourth amendment rights were violated - you made the choice to communicate in such a way that any number of people could have heard you. Subjectively, you couldn't have reasonably expected your conversation to remain private and objectively, society does not generally regard shouted messages as private. On the other hand, if you have a conversation in your home then it would be a violation of the fourth amendment for the police to listen in without a warrant because a person does not expect to be overheard in one's home and society considers one's home to be a private space.

The point isn't whether or not the police were technically capable of listening in. The point is that in order to claim that an act was private you actually have to take some steps to demonstrate that you want and expect it to be private and that society in general would expect it to be private. If you seal a letter in an envelope then you clearly want the contents of that letter to be private. On the other hand, if you write something on the outside of the envelope where any number of people can see it, you have a much more limited claim on how that information is used.

The SCOTUS disagrees with you. There is no such requirement to "take steps" to have an expectation of privacy. The deciding factor is if the situation is such that the "community in general" would expect privacy to exist in the type of situation being examined.

A good example is how the SCOTUS ruled that there was an expectation of privacy regarding IR cameras. Very few houses have any kind of real shielding against IR leakage, but the use of IR cameras was still rare enough that it would be unreasonable to expect the average citizen to take specific steps to defend against that intrusion. The average citizen expects they already had a reasonable amount of privacy.

The implication, of course, is that this is something that can change over time. If IR cameras became commonplace, it would be reasonable to assume a lack of IR privacy. At that point, it would indeed be prudent to expect people to take additional steps toward securing their privacy.

This "community standards" distinction is particularly relevant re: data mining, because relatively few people have any idea how powerful the table JOIN can be, and as such DO expect a certain amount of privacy regarding their data.

The phrase "take steps" was a bit imprecise. My intention was to say that by having a conversation in a private space, such as your home, you're putting yourself in a position where privacy would be expected.

Well, no, that phrase does not actually inherently have that meaning, but rather it's one possible use of that phrase, another valid interpretation would be the plain english one. Given that AFAICS noone was proclaiming an expectation of privacy in that specific sense, your use in that sense was rather unmotivated, which is why I went with the alternative interpretation.

Now, if I had known that you were using the phrase in that specific sense, I'd simply have asked: Where did you get the idea that anyone has an expectation of privacy (in the legal sense), and what does that have to do with the topic at hand (which is people's expectation of privacy (in the non-legal sense))?

I think you are completely confusing "what the law (possibly) is" with "what the law should be". This discussion is about what the law should be, but all you seem to be writing is "but the law is this!". This whole discussion exists because the law as it stands is defective, and people are aware of it (or maybe the law is actually fine, but then it at least tends to be mis-interpreted), so your pointing out what the law is is kinda besides the point. My actual expectation is that the postal service does not archive letter meta data and I have good reasons for that expectation. If the current law doesn't mirror that expectation, then the law needs to be changed.

Now, if you think the law is fine as it is and as it is applied, you maybe should bring forward some argument for why the law is good as it is and why a change would be bad rather than just state what the current law is.

Also, one more point: I would guess that society in general would actually not expect their letter meta data to be archived, and just as it is convention to not open a sealed envelope (a very strong convention indeed, given that it is law, but still it's just a convention in the sense that there is nothing inherent in an envelope that makes the content private - it doesn't really hinder access to the content at all, it's just that for good reasons society at some point has decided "a sealed envelope is not to be looked inside even though it's trivial to do so" when it just as well might have decided instead for a convention of "every sealed envelope is to be opened by the postal service and the content copied and archived"), it should for similar reasons be convention to not archive letter meta data even though that also is trivial to do, and if that is the convention, then all you have to do to "demonstrate that you expect privacy" is to post a letter that is not explicitly labeled "please archive!", just as you "demonstrate that you don't want to be shot" by going outside without a sign saying "please shoot me".