If you write an ISO to a flash drive is it read only? Obviously optical media is inherently read only, but I haven't owned a machine with an optical drive in almost fifteen years and I don't have confidence any way of running an image (aside from a hypervisor) would preserve that property.
Hmm I don't think they are cryptographically protected, it's a security by difficulty thing (obscurity?) - iso is not a "simple" format so corrupting is not simple, either.
On the other hand as you said, physical media can have a strong(er) security guarantee (write once at the physical layer), and iso's are easier to burn than disk images...
Optical media can still be bought and drives still work fine...and there are other advantages to optical media (such as longevity) that as of yet drives can't replicate.
