They do subtract as an accounting identity, but reasoning from that makes no sense. My net income can be calculated as how much I consume plus my net savings. I = C + NS. So withdrawing from savings "subtracts from net income" in the same sense - ie, it has no explanatory power. If I do a home remodel, spending a bunch of saved up money, the way to understand that situation is not "my income would have been higher if I had not withdrawn from savings".