It is pretty unbelievable for me to comprehend how the United States, with absolutely no space fleet can impose a "no-fly-zone" on the moon. But let's assume for a moment, that they could impose their will - why would they? Are they trying to keep something secret?

The U.S. government claims: NASA's "recommendations" of no-fly zones are for preserving and protecting Apollo missions' historical sites and artefacts.

"Apollo 11 and 17 sites [will] remain off-limits, with ground-travel buffers of 75 metres and 225 metres from each respective lunar lander,” states the July 20 guidelines of NASA. Science journal had obtained the guidelines.

The Apollo 14 lunar module (LM Antares) and the Apollo Lunar Surface Experiment Package are visible in this image (note the astronaut tracks between the two artifacts). At the current altitude and lighting the descent stage is clearly visible with its angular shadow (right) and shadow cast by leg (near arrow tip). The LROC NAC image data has not been calibrated, the faint vertical stripes are a natural part of the image and will be removed later after the full suite of calibration data is collected during the commissioning phase. Source: esciencenews.com

According to NASA: There are more than 36 historical sites on the moon.

China and Europe both have space programs and are planned for manned missions to be moon. Is there some purpose, other then historical, why NASA doesn't not want these sites investigated?

An article appearing in The Hundu under the byline of R. PRASAD states:  Though these restrictions may appear preposterous, there are clear scientific compulsions to collect and study them. For example, studying the discarded food will reveal the viability of bacteria on the moon and, if present, how they have mutated and survived after years of exposure to solar radiation.

Ironically, the sharpest images of the Apollo 12, 14 and 17 moon landings sites were recently photographed by NASA's Lunar Reconnaissance Orbiter (LRO).

Source: http://www.thehindu.com/sci-tech/science/article2432999.ece