Threading over a list of lists of lists and a list of atomic expressions

Although I solved the following problem using MapThread, I'm curious about why Thread does not work as I expect it to here.

Given two lists of length n, where the first list is a list of lists of lists and the second is a list of atomic expressions, such as:

list1 = {{{1, 2}, {3, 4, 5}}, {{1, 3, 5}, {9, 8}}};
list2 = {"a", "b"};

and where list1[[n]] can contain 1 or more lists of atomic expressions. I would like to create a single list that places the nth element of the second list at the end of every sublist of list1[[n]], producing:

{{{1, 2, "a"}, {3, 4, 5, "a"}}, {{1, 3, 5, "b"}, {9, 8, "b"}}}

I first tried the following

appendTo[ll_, item_] := Map[Append[#, item] &, ll];
Thread[appendTo[list1, list2]]

But this returns:

{{{1, 2}, {1, 3, 5}}, {{3, 4, 5}, {9, 8}}, {{"a", "b"}, {"a", "b"}}}

even though threading an undefined function seems to produce the right expressions:

Thread[f[a, b]]
{f[{{1, 2}, {3, 4, 5}}, "a"], f[{{1, 3, 5}, {9, 8}}, "b"]}

MapThread works fine:

MapThread[appendTo, {list1, list2}]
{{{1, 2, "a"}, {3, 4, 5, "a"}}, {{1, 3, 5, "b"}, {9, 8, "b"}}}

So why is Thread producing the behavior shown above instead of giving the same result as MapThread?

Threading Rule on the same lists also produces what I expect:

Thread[Rule[list1, list2]]
{{{1, 2}, {3, 4, 5}} -> "a", {{1, 3, 5}, {9, 8}} -> "b"}

Answer

The simple answer: Thread does not have the attribute HoldAll, so its contents are executed before it processes them.

But, it is not quite that simple. If you add HoldAll, you still don't get what you want

Internal`InheritedBlock[{Thread},
 SetAttributes[Thread, HoldAll];
 Thread[appendTo[list1, list2]]
]
(* {{{1, 2}, {3, 4, 5}, {"a", "b"}}, {{1, 3, 5}, {9, 8}, {"a", "b"}}} *)

which implies that in this form only the first list is threaded over.

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Note, I used Internal`InheritedBlock to retain the behavior of Thread while not allowing my modifications to leak.