[ajb]

The flash chip usually has a write enable/disable pin and most monitors and TVs will wire it to prevent writes to the EDID. I would guess only cheap ones don't bother. It's risky as without protection, a voltage glitch during a read can turn it into a write and trash the flash.

[nyanpasu64]

Attaching the write-protect pin to +V is literally free in the PCB design process; IMO not doing so is a design error or decision (though IDK how much thought was placed into allowing users to rewrite the monitor identification).