Actually they ARE sent straight home (well, given how remotely located we were it took a night or two to get them back to Sydney but from there it was a flight to LA and on to where-ever they lived). The contracts they sign (and all the crew members sign) are brutally clear on the confidentiality front. If the show can prove you gave information about the show to ANYONE, you are liable for $1,000,000. And it's an airtight contract.
Now, I am assuming if you're Laura or one of the girls that went home 1st, your family is going to figure it out...'hey, honey, why were you only in Austrlia for 4 days?' but when it gets down to the last 5 or so, they've been gone long enough that - unless you have the shoot schedule in hand - it's hard to know who placed where.
And by the time the public develops an awareness of the show, the shoot is over and ALL the girls are home - so it doesn't help them guess either.