Actually, Wolfram Alpha is doing a sensible thing in each case: it is not adding the temperatures, it is comparing them, resp. calculating the difference, when you give different scales. You can see that in the output, where it says "difference".
I'm not sure about the confusion... the question to W|A is not well-defined, but lets say someone asks you "2 + 4?" and you answer "the difference is 2". Does it make any sense? Not to me.
What if someone writes you the question "what is 12, Park Avenue + 34, Park Avenue?" Wouldn't you assume that this cannot be about addition, and instead tell him who lives in those two places and perhaps the walking distance?
The confusion is all on your side.