It's probably a narrower aperture as you zoom in (larger f number).
This isn't the camera choosing the aperture as much as a physical limitation of the lens, the aperture can't remain as large when zoomed in, there isn't the room inside the lens for this to happen.
You don't need to do anything about this, just zoom and shoot even in Manual mode, if autoISO is on then you will get a correct exposure until you run out of light. If you do get an underexposed result then that will be the time for you to do something about it by either slowing the shutter speed, or upping the ISO if it's not on auto already.

As Paul said, zoom lenses with constant aperture are almost always in the pro sector, this is probably so that said pro photographer knows his/her aperture will not change with focal length, this presents a more difficult design, probably higher quality components and a hefty price tag.
This is not a limitation of P&S, both of my Nikon SLR lenses do this as do most zoom lenses.
Bottom line is, set the focal length and press the button.

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