That's a hypothetical example, and it would of course be impossible to know what specific piece of data led Google to present a specific ad, and whether they're

That's a hypothetical example, and it would of course be impossible to know what specific piece of data led Google to present a specific ad, and whether they're involving an analysis of how historical interests relate to current interests. Whether Google actively engages in such analysis now or not, they easily could. I personally use AdBlock so I don't see Google ads very often, but I have been creeped out by ads in the past, and I have had Google infer interests in changes to relationship status based on mail content (e.g., offering engagement rings). I believe they have manual filters to try to stop inferences that are too offensive from being made (e.g. suggesting a divorce), but those, of course, can never be perfect, and it doesn't mean they aren't making the inference or maintaining the data necessary to do so; it just means that they are blocking it from showing up. I'm not really saying this is either good or bad. The question was why people care less about reading emails to filter for spam than they care about people reading emails to develop a consumer's ad profile. These are some reasons why.