Reference:
That's exactly the problem. They all get magazine deals of some sort and their agents rely on continued media exposure to get them. Treating walkers as though they no longer existed in the eyes of channel 4 would definitely make them think twice in fact I doubt they'd try to pull the stunt in the first place. That would mean walking had very negative connotations and I think the only people who would walk would be the ones who genuinely just had to get out for the sake of their sanity (i.e. maybe 1 per cent of all walkers).
I think those that constantly talk of walking but never do it are more disruptive than those that walk - but the talkers (who hold the show to ransom for weeks on end) are rewarded, rather than punished. Sophie Reade kept talking about leaving last year because she was bored and missed going out with her friends: WINNER!!! Siavash, withdrew himself from the game - sort of walking for those too idle to get up and go: RUNNER UP!!!
Your
negative connotations might have convinced Laura to have stayed - but i think we saw enough in 3 days to suggest that she wasn't going to be funny, entertaining or controversial. In this instance, rather than denying another housemate the chance; such a quick departure actually opened up an opportunity - which Sam grabbed with some effect! I wonder how many among the legion of Sam fans (currently 90,000+ Twitter followers) are now disappointed that Laura walked?