"They could blame the weather on the TSA if they wanted to."
Well said!

My own experience with this has been:
1. An Alaska Airlines flight attendant, who said that it was US law for inbound international flights. Only passengers in the forward cabin can use the forward lavatory. Period. Maybe he was lying; maybe not.

2. A United A320 with an inoperable forward lavatory that was not allowed to fly because the flight crew can only use the forward lavatory. This is clearly not the case on the E-135/140/145 when they have to go to the back, and there are inbound international flights on these aircraft types. So I'm guessing it's United's rule.

3. Profoundly aggressive response by Continental subsidiary flights when some passengers have tried to use the forward cabin on inbound international flights. They come out of the woodwork. I've not seen this on United subsidiary or other US airlines.

Indeed, the announcements used to say, "It is United's policy..." and now the announcement doesn't attribute authorship.

I just think it would be much more honest to say, "The forward lavatory is reserved for our first class customers." No need to invoke a "security policy." And then use common sense if someone from the "B" cabin or anywhere aft tried to go forward.