I think the reason this seems surprising is the value/reference semantics of values. For example:

    def foo(i, j):
      i[0] += 1
      j += 1
    a = [0]
    b = 0
    foo(a, b)
    print(a) # [1]
    print(b) # [0]

So people think the body of the loop like a function call.

I don't think it's a bad expectation, in fact I think it's quite a natural expectation—in particular if you've programmed functional languages where modifying values is the exception, not the rule—which is why it surprises people. It's just not the one way Python chose.