I don't know if you've been reading the forums or not but I see MANY threads on why does this app crash and "my iphone is really hot running iOS 5 what gives".
So to respond to your statement.. NO a lot of people don't know what a beta is....
I think some of that has some from the gaming industry with often includes "beta access" for pre-orders or those types of things and these betas are really just early looks at games and maybe some stress testing of servers (though when it is server stress testing you hear people cry bloody murder that they can't "test" the game because it is too laggy and ignoring the fact that the lag is probably giving developers very good server data).
iOS5 Beta is very different then the "Gears of War 3" beta that just ran on xbox live and many people don't know the difference.
I work in the IT field, I know what a Beta is, I know what I was getting and I know what to expect. I may mention things that are not working but will never complain about stuff that isn't working in a beta and would certainly never rate an app negative for lack of iOS5 compatability.. I mean the dev just got access to the beta themselves so how are they supposed to have it ready for iOS5 yet? I was doing something in facebook today and my phone rebooted... I will report it to Apple, but I am not going to go post a review on the facebook app page about the app being horrible and rebooting my phone. I have beta code running and that comes with the territory.
I like the suggestion that if you are not running a beta iOS or iTunes you cannot rate apps.