The flight ban is a minor issue. The reasons for the flight ban are not.
The problem is that the oil spill releases vapor. Said vapor has two nasty tendencies. The first is that it makes people woozy so that they make bad decisions. The second is that it can explode when exposed to fire, such as may be found in airplane engines.
Unless you understand how to avoid the risks, flying low over an oil spill is a really good way to get yourself killed.
(Incidentally the observant may wonder why the workers aren't being provided with masks. They weren't until people complained. Now they have the masks. But unofficial reports say that BP is threatening to fire people who use the masks. This fact alone shows sufficient hypocrisy that I'm expecting BP to dodge all responsibility as fast as possible. Just like Exxon did when they got over 3000 lawsuits over people sickened by fumes from the Valdez spill thrown out because "they just had flus and colds". Yet another reason for them to not want photographic documentation of the cleanup efforts.)
Update: See http://www.adn.com/evos/stories/T99032316.html for background on how Exxon dodged health claims, and http://www.huffingtonpost.com/2010/05/26/gulf-oil-spill-clea... for confirmation that there are health problems with the cleanup this time.