Matt,
Some national events and other big races have stopped posting frequencies to avoid potential sabotage. The last big race I went to, it was announced and we all knew we were supposed to go check with the frequency control/radio impound guy prior to our race. The problem with this is that people don't hear the announcement, don't listen, don't think some stuff applies to them and count on things always being the same, or at events like this, some people spend time elsewhere like their hotel room or sightseeing when they are not racing.
I kind of think this is a good idea. I have been to way too many big races and heard people say "so and so turned their radio on". It is definitely welcome in 1/8 onroad when we have $3000 wrapped up in our cars and it really sucks to have a runaway. I can also see it being desired at an event like this too since there is a national title on the line and it does tend to make people act funny.