Because this is not fault from fdisk point of view. We do not install bootloader with fdisk, we could even just throw away this read at all, except it is "useful" to fill up the space outside the partitions.
For boot program install, the only way since 2005 is installgrub and in illumos for last 2 years - installboot.

That's absolutely fine, but I would like to recommend an another solution:
If the user explicitly specified a master boot file in the command line then

it would be better to not return silently, if that file cannot be opened.
Example (snippet):
.
.
.
#define DEFAULT_MASTER_BOOT_FILE    "/boot/pmbr"
.
.
.

    /*
     * If the master boot file hasn't been specified, try to use our
     * default.
     */
    if (io_mboot == NULL) {
        io_mboot = DEFAULT_MASTER_BOOT_FILE;

        if ((mDev = open(io_mboot, O_RDONLY, 0666)) == -1) {
            /*
             * This is not fault from fdisk point of view.
             * We do not install bootloader with fdisk,
             * so just throw away this error and return.
             */
            return;
        }
    } else {
        errno = 0;
        if ((mDev = open(io_mboot, O_RDONLY, 0666)) == -1) {
            int errno_sav = errno;

            (void) fprintf(stderr,
                "fdisk: Cannot open master boot file '%s' : %s.\n",
                io_mboot, strerror(errno_sav));
            exit(1);
        }
    }

...

I did add this one, this time. Next time, just write it down and post it for the review and RTI. Not joking at all. Those are exactly those small updates which do change the world.
we do not need to save errno there btw, because strerror() is called before fprintf() and the value of the errno will not change after open().

Nice find, please open the issue and post the review, lets get them fixed:)

noone is daring to touch those license lines. I'm certainly not going to:)

Ok, this should be part of this issue: Bug #1216: long lines in man pages

(other related: Bug #9103: opengroup acknowledgement should be properly formatted in man pages)

oops:)