That's not out of any spatial analogy, but rather because many apps interpret mouse-wheel movement (perhaps chorded with some modifier key) as controlling some number, where up means "make number go up" and down means "make number go down."

VLC, for example, puts volume on the scroll wheel, such that WheelUp means "volume goes up."

Even Operating Systems themselves do this: if you can find a program with an OS-native number-picker control — the old-school kind with the little up and down arrows on the right of it — then if you have said control focused, WheelUp will increase the value, and WheelDown will decrease it.

IE6 decided that it — and all browsers coming after it — should do the following: Ctrl+WheelUp means "page elements get bigger", while Ctrl+WheelDown means "page elements get smaller."

Note that while this gesture (also accessible as Alt-Plus/Alt-Minus) was called "zoom" in IE6 and onward, it was never meant to be spatial zoom — the page isn't getting closer to you; you're not shrinking the viewport relative to the page; you don't get scrollbars where you didn't have them before. Rather, this gesture is literally increasing the base font-size: the page elements are growing — and then being reflowed, as fewer of them now fit inline in the viewport. So the mouse wheel here is controlling content size. Ctrl+WheelUp here means "content size up." It's an accessibility tool, not a "navigating within the page" tool.

Google Maps — released not long after IE6, and long before the first browser (Mobile Safari) shipped with spatial-metaphor zoom — decided to just "extend the metaphor" of the browser it was operating within.

Google presumed that users would guess that the browser's "change the content size" shortcuts might do something useful in Google Maps. They presumed that users would have a mental model where, if you "blew up" the page by increasing the content size, then a web-app could maybe notice that, and respond by doing something smart and analogous, e.g. re-rendering the content at a higher zoom level instead.

If they could have, Google likely would have directly overridden the accessibility "content size" shortcuts (which are pretty useless on a mapping app) and mapped instead them to zooming the content. But because browsers have no API for overriding browser-chrome accelerators on a per-tab basis, the best they could do instead was to make analogous shortcuts: just as Ctrl+WheelUp increases the content size, plain old WheelUp in Google Maps increases the map view zoom as if to increase the content size. (Helpfully, these analogous gestures would also be discoverable — people tend to be pretty sloppy at timing holding of modifier keys correctly, so they'd mess up and accidentally activate the correct gesture while trying to activate the incorrect one!)

---

Google Earth — released a few years later — also uses WheelUp to mean "descend"... but for an entirely different reason.

Google Earth, with its various navigation tools, uses cursor-key UpArrow, and also "click and drag the viewport upward", both to mean "zoom in" / "descend toward Earth." It's pretty natural-feeling in context.

Google Earth is always showing you a tilted view of the Earth, with the view curving away to the horizon when zoomed in, or on a tangent to the atmosphere when zoomed out. In either case, the Earth's surface is always aligned with the bottom of the viewport – and so going inward is always the same as going downward, toward the Earth.

WheelUp is "zoom in" here by analogy to the other uses of "up" to mean "descend." They're all unified in meaning "grab the stuff in the viewport and move it up, such that the camera moves down."

But note that this UX was also designed before inverted-scroll touchpads. The "click and drag upward to descend" gesture has exactly the same intuition behind it as double-tap-dragging upward does on an inverted-scroll touchpad. In both cases, they're meant to evoke a sense of sliding the content around in the viewport, rather than moving the viewport.

As such, presumably, had Google Earth been designed today, they'd likely have made WheelDown descend — as that would have resulted in the correct touchpad gesture (swiping up with two fingers to descend), rather than an inverted one (where you must swipe downward with two fingers to descend.)

But then again, nobody with an inverted-scroll touchpad is swiping to zoom; they're pinching to zoom. So maybe they'd have left swipe-to-zoom the way it is, so that it would still make sense for people zooming with a mouse wheel.