Area 51 Mystery - why is it not shown on google maps ?

Recently i saw few videos about Area 51 on you tube which led me to do some quick googling about the same and i was surprised to get lot of info.

1) On Google Maps the entire area 51 region is blocked or shown with gray color. Even we can see pentagon aerial view but why is area 51 blocked?

http://www.google.com/maps?hl=en&tab=wl&q=Army%20Navy%20Dr%20%26%20Fern%20St%2022202

This is pentagon aerial view you can even see the satellite image. But try this,

http://maps.google.com/maps?ll=37.240203,-115.818558&spn=0.11,0.18

As shown in the maps, Area 51 is totally removed. Even on microsoft local search it is removed.
http://local.live.com/default.aspx?cp=37.240203-115.818558&style=r&lvl=13&v=1

If you want to read more about Area 51 visit here,

http://en.wikipedia.org/wiki/Area_51