It just seems a bit overpowered in at least one case (the Warlock 1st level armor spell). And given how freely the they seem to be given out, I think with resistance THP can have a huge impact. And I think it wouldn't be unreasonable to call THP analogous to the "magic aura" (especially when they are in some cases coming from a magical effect).
So given that it's not explicitly stated, and that the most obvious reading may lead to some over powered cases, I think it's worth asking.
