Pat Cheney wrote:

What this then means is that any camera with a combined AV/USB port, by the
very definition, MUST be using a proprietary connector!

No, it would be quite possible to design a custom connector that could
accept both standard mini or micro USB plugs, or a custom A/V plug. They
would put the contacts for the AV connections further back on the
connector. It's been quite common to use dual purpose, dual level
sockets on ultra-small Japanese notebook computers. Other variations
have been a USB connector with a small hole to the side of it so an
external, higher power-consumption, USB interface drive could be plugged
only into that USB port (and that USB port being able to supply more
than 500mA).

There's all sorts of creative ways that they could have handled this,
but it was cheaper to use an off-the-shelf connector, and order custom
cables. How many sales do you think they lost because of this custom
connector? Probably zero.