I think its because beta implies 3 things
1)its a preview a head of retail - retail could be months off and demos generally appear a month/few weeks before release
2)its semi exclusive -
3)its time limited so its best to act now, adds a sense of urgency to getting to play it
I would also take a guess that it adds the feeling for some that it makes it look like the devs are (well, could be) caring about what is said about the game so that it could get fixed before gold.
Personally coming from the land of PC's where games (and software) regularly ship with the moniker of beta or alpha and DO have broken functionality/incompleteness for people to leave comments on or report on I find it annoying that this fad in the console world carries over when people inevitably come to look for PC software and see beta or alpha and expect it to be pretty much all working as it is delivered on consoles. As a result of stuff not being how they expected it to be they then persist in slamming/spamming the dev forums whining about it. Granted PC games are often guilty of this (MMO's in particular) saying "open beta weekend" when it really means "stress test", as a result you again get forums of moaning saying OMG LAG.
Saying all that it sometimes works the other way for people who aren't that interested and think "beta - i'll leave that till its finished", this is why i pasted the news article in the other reply which pretty much says its a demo, not a beta, beta time is over. So for those interested in playing the final thing now go ahead, ignore the stupid PR stunt.