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Price. Why spend $4 where $0.05 will do, and will likely never fail anyway? It's a last ditch protection system, not really something that should be tripping all the time.
That said, it used to be fairly common to have breakers on a couple circuits (headlights, for one).
In addition to cost, I'd suppose reliability might be a second consideration. Breakers have contacts in them, which with enough vibration and temperature/humidity cycling might fail, I guess... whereas a fuse has none.
What else.. fuses are faster, better current breaking capacity for DC (at least at this price point). I'd guess the tempco might be better too. (the trip point both devices moves with ambient temperature).
That's how it generally works already. Important stuff is on one CAN bus (ECU, ABS pump, auto trans controller if it has auto trans, airbags, etc). All the secondary stuff like door modules (controls locks, windows, etc), cabin illumination, the radio/navi and whatnot are on a secondary CAN bus (or LIN, or..).
This way if your rear door module dies and manages to take down the (secondary) bus, the car still runs.
I don't see much point in securing it, as you need physical access anyway. I'd rather see it go the other direction, standard, open interface, instead of each manufacturer using a proprietary communication scheme. (CAN only defines lower layers).
This is like suing computer makers for people being able to hack a computer they have physical access to. It's not possible to prevent.
That's also how ethernet works... differential transmit and receive signals, transformer coupled for isolation. (to prevent ground loops and such).
Shutting off all the electronics on a fly-by-wire plane seems like a fairly bad idea.
Here's the rules, FWIW. Pennies are only valid to 25c.
(2) A payment in coins referred to in subsection (1) is a legal tender for no more than the following amounts for the following denominations of coins:
(a) forty dollars if the denomination is two dollars or greater but does not exceed ten dollars;
(b) twenty-five dollars if the denomination is one dollar;
(c) ten dollars if the denomination is ten cents or greater but less than one dollar;
(d) five dollars if the denomination is five cents; and
(e) twenty-five cents if the denomination is one cent.