This is not a bug, this is by design. And, yes it's set to default, not 255 (255 == -1). If you do not like this - submit your request to
Broadcom
it is set to 255
[admin@(none) root]$ wl txpwr1
TxPower is 51 qdbm, 12.75 dbm, 19 mW Override is Off
[admin@(none) root]$ wl txpwr1 -1
[admin@(none) root]$ wl txpwr1
TxPower is 127 qdbm, 31.75 dbm, 255 mW Override is Off
[admin@(none) root]$ wl txpwr1 -m 29
[admin@(none) root]$
No, as this would break much of internals and I do not have time to debug whole the crap. Also, this would cause lot of complaints from other users, as they remove colons already.