"They say you shouldn’t be allowed to modify the code in your car because you might defraud a used car purchaser by changing the mileage."
All you would need to do is checksum to know if the consumer had manipulated the code. Let car owners see the code running on their cars but void liability if the software has been altered.
Plus, the idea of a car needing a net connection to operate is a non-starter (excuse the pun).
Your turn.
I implement a RSA key for each car, signed before it leaves the factory. Each key is housed in memory that is heavily epoxied and very difficult to remove (in a place that you would have to remove the majority of the engine to reach). The cars ignition system requests validation from the on board key on every start (in fact it gets the actual <start car> command from the chip itself). If the key is not present, the car does not start and instead tells you to return it to the dealership.
Now you say, I can remove the key and then reverse engineer it. What if I put n keys? How many do I realistically have to add to make it so time and labor intensive for you to remove my little inexpensive keys that you just give up? Probably not that many. Oh and did I mention that all of them are in an array? Better be sure you didn't miss one or it will zero itself out giving you no hope of recovery. (The warranty specifically mentions this as something that will void it)
After all that, if you still want to attempt to modify it. Fine. I'll add 1% to the purchase price of the car and reimburse anyone who bought lemons from your little (illegal) scheme.
Requires no Web connectivity of my year 2000 Nissan Micra.
Alternately, the odometer could (and I'd argue should) be implemented in hardware, possibly as an ASIC (or maybe FPGA) plus dedicated EEPROM to store the counter, wired directly to the relevant sensors. Perhaps some data interface with the main computer, but it should be read-only. That would solve this issue right quick.
"Cars should have no anti-tamper mechanisms" != "Owners should be legally able to modify their car."
Even changing the mileage is not a crime, the crime occurs if you sell it and commit fraud by lying to the buyer.
Once again, not illegal to make the change, but probably fraud if you were to lie to your insurer about it.