Why fdisk can't open root disk with MBR for writing?

[Yuri]

I am trying to change the active partition in MBR.
This should be a matter of changing only two bits (clearing one in one 
byte and setting another one in another byte).

However, fdisk complains:
fdisk: Failed to write MBR. Try to use gpart(8).

truss reveals that fdisk failed to open the root disk for writing:
open("/dev/ad4",O_RDWR,00)                       ERR#1 'Operation not 
permitted'
open("/dev/ad4",O_RDONLY,00)                     = 3 (0x3)
...
...
pwrite(0x3,0x34048200,0x200,0x0,0x0,0x0)         ERR#9 'Bad file descriptor'
Failed to write MBR. Try to use gpart(8)

The question is: why the disk can't be open RW under root?
The secondary issue is the bug in fdisk: once -a option is supplied and 
it can't open it RW it should just say so, and not suggest using gpart 
because gpart will probably have the same issue.

Yuri