Of course argv is mutable since it's a local variable/param. But what if the pointers are placed, by the OS, into read-only VM pages? Begrudgingly, the string data is made writable though, to conform to the standard.
I don't know of any platform where there are any ill effects. But the number of platforms out there with C implementations is much larger than my direct experience. Maybe this is just some historic baggage that is overdue for a revision. It was discussed in 1998, but no changes were accepted at the time.
In most coding situations, I probably wouldn't care, the same way I don't care about, say, platforms where for some reason you cannot compare pointers to distinct objects using inequality (another thing not required by ISO C). It's better to break the rules knowingly, though. You want to be badass, not dumbass.