Why is the government is blocking access to oily beaches?

I’ve been reading stories about the government at every level blocking access to areas affected by the oil spill pretty much since the oil spill started. Just this weekend, Duncan Davidson reported on new restrictions on journalists documenting the effects of the spill in, Should it be a Felony to Cover the Oil Spill? Glenn Greenwald rounds up news of law enforcement officers working with BP personnel to prevent journalists from covering the spill. He also notes that this pattern of behavior precedes the spill — law enforcement previously detained a freelance photographer for photographing a refinery in Texas City, Texas. The reasons why the government might not want people to see how bad it is on the Gulf coast these days are obvious, but the calculus is depressing. The government sees the costs of harassing journalists who are trying to document the spill as being lower than the costs of people seeing the effects of the spill.