Strange I've never noticed that before. It isn't in the man page. It is mentioned in my redhat man page as an extension to the origonal BSD source. Seems to be working as expected, perhaps the OS X man pages need revising.
This syslogd(8) has a syntax extension to the original BSD source, that makes its use more intuitively. You may precede every prior-
ity with an equation sign (``='') to specify only this single priority and not any of the above. You may also (both is valid, too)
precede the priority with an exclamation mark (``!'') to ignore all that priorities, either exact this one or this and any higher pri-
ority. If you use both extensions than the exclamation mark must occur before the equation sign, just use it intuitively.