I see it as a sign that JSON is not that great for structured data if people prefer to pack their datetime structure into a string rather than the more native object format.

Oh, gotcha. Well, I definitely agree it would make more sense for it to be an object like you mentioned but I think it's probably more explainable by the fact there is native support for ISO date / time strings in most programming languages, making it just as easy to parse a date or time from a string value and having the added advantage of reducing the size of the payload on the wire. I could be wrong